Something is wrong
Second monitor not detected
Two different faults hide behind the same complaint. A monitor that says "no signal" has been found by its own electronics and is waiting for a picture; a monitor that never appears in display settings has not been found at all. They have almost nothing in common, and the first step here is to work out which one you have, because it decides everything after it.
Read what the machine thinks it has
The run, in order
Read what the monitor itself says before touching the computer
The split that decides which half of this page applies. Press the monitor's own menu button: if its on-screen menu appears, the panel and its power are fine and only the picture is missing. A monitor showing "No signal" or "Check cable" has working electronics and is telling you it sees nothing on the input it is watching — which is very often the wrong input rather than a missing one. A monitor that shows nothing at all, menu included, is a power or panel fault and none of the computer-side steps below will help it.
Change the input source on the monitor
Monitors do not reliably switch to the socket you plugged into, and many will not scan the others at all once they have settled on one. The button is usually marked Source or Input, sometimes hidden in the menu. Step through every input, slowly — some monitors need several seconds on each before they decide. This costs nothing and is the single most common cause of a screen that is connected correctly and shows nothing.
Set the display mode to Extend
Windows keeps a separate setting for what to do with a second screen, and it survives reboots and cable changes. Press Windows and P together: the options are PC screen only, Duplicate, Extend, Second screen only. On PC screen only the monitor is detected, powered and deliberately sent nothing, which looks exactly like a fault. On macOS the equivalent is the Displays panel, where a screen can be mirrored or arranged off to one side where nothing ever lands on it.
Drag this page onto the second screen and read what arrives
The useful thing here is not the number but which machine answers. Move the browser window across to the second monitor and the page reports the resolution, refresh rate and colour depth of the screen it is now on. A different set of numbers from the first screen proves the operating system has the monitor, is driving it, and knows what it is — so anything still wrong is a setting or a cable running below its grade, not a missing display. If the window will not move across because there is nowhere to move it to, the monitor is genuinely not detected and the steps below are the ones that matter.
Make the system look again
Both systems have a button that forces a fresh scan rather than waiting for one. Windows: Settings, System, Display, scroll to Multiple displays and press Detect. macOS: System Settings, Displays, and hold Option — the button in the corner becomes Detect Displays. Worth doing after every change below, because a monitor plugged in while the machine was asleep is often simply never looked for again.
Check that the cable carries video, not just power
This is the modern version of the fault and it catches people who have done everything else right. A USB-C cable that charges a laptop perfectly may carry no video at all: DisplayPort Alt Mode is optional, and charging cables that omit it are common and look identical. The same trap exists in Thunderbolt docks and in cheap HDMI-to-USB-C adapters that only work in one direction. Borrow a cable known to carry a picture before suspecting anything expensive. While you are there, try a different port on the computer — on a desktop, one on the graphics card rather than the motherboard, which is a dead socket on most machines with a separate card.
Docks and adapters that state HDMI or DisplayPort outputPaid link: we earn a commission if you buy, at no extra cost to you. Borrowing a cable that is known to carry a picture costs nothing and settles it either way, so try that first.
Check what the port and the cable are rated for
A link that is merely short of bandwidth does not degrade politely — it drops out, flickers, or refuses the resolution entirely, which reads as "not detected". 4K at 60 Hz needs HDMI 2.0 or DisplayPort 1.2 at minimum, and a high refresh rate at that size needs more. Older or unbranded cables often run below their printed grade. If the monitor appears at a low resolution or a low refresh rate and will not go higher, you have found the limit rather than a fault, and the fix is a cable or a port that is rated for it.
Find out how many screens the computer can actually drive
Graphics hardware has a fixed number of display pipelines, and it is often lower than the number of sockets. Many laptops can drive two external screens and no more; some can drive only one, whatever the dock suggests. On a laptop with both integrated and separate graphics, the external ports are frequently wired to only one of them, and which one changes what is possible. A third monitor that will not appear while two work is usually this limit rather than a fault, and the manufacturer's specification page is where the number is written.
Once it appears, check what it is actually running at
A monitor that was hard to detect is often running below its rating afterwards, because the system settled for whatever the link would carry. The measurement is worth taking on the second screen specifically: a panel sold as 144 Hz that measures 60 says the cable, the port or the setting is still holding it back, and that is a different and much cheaper problem than the one you started with.
Why the first step comes first
Almost everything written about this problem starts with cables. That is the right advice for one of the two faults here and wasted effort for the other, which is why this page starts by telling them apart.
A monitor whose own menu appears has power, a working panel and working electronics. It is not broken and it has not failed to be detected — it is waiting for a picture, and the question is which socket it is watching and what the computer has been told to send. That is steps two and three, and they are free.
A monitor that never appears in display settings is a different matter. The machine has not found it, and the chain between them is cable, port, bandwidth and graphics hardware, in that order of likelihood. Those are steps six to eight.
The test in step four is what separates the two for certain: if this page can be dragged onto the second screen and reports different numbers, the operating system has that monitor and the remaining problem is small.
Why this page cannot just list your monitors
It is the obvious thing to want, and it is worth saying why it is not here. A page in a browser sees one screen: the one its window is on. window.screen describes that screen and nothing else, which is why step four works by dragging the window across rather than by reading a list.
There is an API that does more — Window Management, which can enumerate every display attached — but it is Chromium-only, it asks the visitor for a permission first, and a permission prompt is a poor trade for a list that the operating system’s own display settings already show. Worse, it would answer the wrong question: it reports what the system has already found, so a monitor the system has not detected is absent from it too. The one case where it would help is the one where it has nothing to say.
So the test here does the thing a browser genuinely can do: it reports, from the second screen itself, what the machine thinks that screen is.
Reading the outcome
The wrong input. The monitor’s menu works, and stepping through its sources finds the picture. Nothing was ever wrong. This is the commonest ending on this page.
The display mode. Detected in settings, nothing on screen, and Windows and P was on PC screen only. One keypress.
A cable with no video in it. Charges, connects, carries nothing. Normal for USB-C, and invisible from the outside.
Not enough bandwidth. It appears, then drops out, or refuses the resolution it is sold for. The cable or the port is below grade. Confirm by dropping the refresh rate: if the drop-outs stop, that was it.
A hardware limit. Two screens work, the third will not, and the specification says two. Not a fault, and no cable will change it.
The one that is not a fault at all
A laptop lid. Closing it sleeps the machine on most default settings, so the external screen goes dark along with everything else, and it looks exactly like a monitor that has stopped being detected. The setting that changes it is three clicks deep in power options, and it is worth checking before buying a cable.
Common questions
The monitor says "no signal" but Windows shows two displays.
Then the computer is sending a picture somewhere and the monitor is watching a different socket. Change the input source on the monitor itself and step through all of them. If the monitor has several of the same kind of port, note that they are not interchangeable — the machine is driving the one you plugged into, and the monitor may be watching another.
My USB-C cable charges the laptop but gives no picture.
That is a complete explanation rather than a coincidence. Carrying video over USB-C needs DisplayPort Alt Mode, which is optional, and plenty of cables sold for charging do not have it. Nothing about the cable looks different. Borrow one that is known to carry a picture, or use the monitor's HDMI or DisplayPort socket instead.
It worked yesterday and today it does not.
Start with the display mode and the detect button, and skip the hardware steps. A Windows update, a dock being unplugged while the machine slept, or a remote desktop session can all leave the display mode on PC screen only. A monitor that was working and then vanished with nothing moved is nearly always a setting.
The second screen appears and then disappears every few seconds.
That is a link that is not quite holding, which is almost always the cable, an adapter, or a port being asked for more bandwidth than it can carry. Try a lower resolution or a lower refresh rate: if the flicker stops, you have confirmed it, and the fix is a better cable or a port rated higher, not a new monitor.
I closed the laptop lid and the external screen went off too.
The lid closing triggers sleep by default on most laptops, and a docked machine needs that behaviour changed. Windows: Control Panel, Power Options, Choose what closing the lid does. macOS: a Mac will run with the lid shut only while on power with a display and an input device attached. Nothing is broken, and no cable will fix it.
Two monitors work but the third one will not.
Usually the number of display pipelines in the graphics hardware rather than a fault, and the specification page will give the number. Daisy-chaining over DisplayPort, where a monitor has a DisplayPort output as well as an input, is the common way around it, and it needs the feature switched on in both monitors' menus.
The monitor is detected but the screen is black.
Detected and black means the system has it and is sending something it cannot show. The usual cause is a resolution or refresh rate the monitor does not accept, set while it was connected differently. Unplug the second monitor, set the first one to a plain 1920 by 1080 at 60 Hz, reconnect, and raise it again from there.