Why will my second monitor not display anything?

An ordered check that separates the cable, the port, the monitor's own input selection and the computer's display settings — the four places a blank second screen actually comes from.

Difficulty: beginner
Time: 25 min
Read: 4 min

Short answer

Check the monitor is on its correct input first, because a monitor showing no signal is usually looking at the wrong socket. Then swap the cable, then try a different port on the computer, then force the computer to re-detect displays. The panel itself is rarely the problem, and a monitor that lights up at all is proving its backlight and power are fine.

A blank second screen has surprisingly few possible causes, and they can be eliminated in a fixed order in about ten minutes. What makes it feel mysterious is that people usually start at the software end, changing display settings, when three of the four causes are physical and take seconds to rule out.

Step by step

Check the monitor is showing its own menu.Press the monitor's menu button. If its on-screen menu appears, the panel, the backlight and the power supply are all working, and the fault is upstream of the panel. This one test eliminates the expensive possibility immediately.
Set the monitor's input manually.Most monitors have several inputs and an automatic selection that frequently picks wrong or gives up too early. Use the monitor's own menu to select the specific input the cable is plugged into rather than leaving it on automatic.
Reseat both ends of the cable, then swap it.A cable that has been nudged out by a few millimetres behind a desk looks connected. If reseating does not do it, swap in a different cable — cables fail far more often than ports.
Try a different port on the computer.On laptops in particular, only some ports carry video. On desktops with a separate graphics card, the ports on the card and the ports on the motherboard are different sets, and only one is usually active.
Force the computer to look again.There is a detect displays option in the display settings, and most laptops have a keyboard shortcut to cycle through display modes — screen only, duplicate, extend, second screen only. Cycling through it is a quick way to shake a display loose.
Rule out the adapter or dock if there is one.Plug the monitor straight into the computer, bypassing any dock, adapter or hub. If it works direct and not through the dock, the dock, its cable or its power supply is the fault.
Test the monitor with something else.Plug a games console, a streaming stick or another laptop into it. If the monitor shows that source, the monitor is proven good and the problem is on the computer side.
Update or roll back the graphics driver.Only after the physical checks. A recently updated driver that broke a working setup can usually be rolled back; a very old one may not support the monitor's resolution.
Check what the computer can drive at once.A laptop or dock has a maximum number of displays and a maximum total bandwidth. If a third screen simply will not appear, or the second only works at a reduced resolution, that ceiling may be the answer rather than any fault.

Tips

Do one thing at a time and put back anything that did not help. Changing three things at once means you never learn which one mattered.
If the screen shows an image briefly at start-up and then goes black, that points at the driver or the display settings rather than the cable — the hardware clearly worked for a moment.
A monitor connected but not detected sometimes appears if you connect it while the computer is asleep and then wake the machine.

Common mistakes

Starting in the display settings — Three of the four common causes are physical. The settings are the last place to look, not the first.
Assuming a dead monitor because there is no picture — A monitor whose own menu appears is working. That single check saves people buying replacements they did not need.

If it doesn't work

Monitor says 'no signal' and goes to standby

Cause: Wrong input selected on the monitor, or nothing arriving on that input — Fix: Select the input manually in the monitor's menu, then check the cable and the port at the computer end.

Nothing at all, no menu, no light

Cause: Power — Fix: Check the lead and the socket, and try a different socket. A monitor that shows no light at all is a power problem and should not be opened — the power supply inside holds a charge.

It works direct but not through the dock

Cause: The dock, its cable or its power supply — Fix: Use a cable rated for video, plug in the dock's own power supply, and update its firmware.

Works until the computer sleeps, then never comes back

Cause: Power management or a driver issue — Fix: Update the graphics driver, and turn off the setting that allows the computer to power down USB devices to save energy.

Detected in settings but the screen stays black

Cause: A resolution or refresh rate the monitor cannot display — Fix: Set the display to a lower standard resolution and refresh rate, then raise it again until it fails, which identifies the ceiling.

Questions people ask

Why does the laptop screen work but not the external one?

Because they are driven separately. The laptop panel proves the graphics hardware is alive; it proves nothing about the external port, the cable or the monitor's input selection.

Do I need a special cable for a high refresh rate?

Yes — refresh rate and resolution together set the bandwidth needed, and an older or unrated cable may work at a low setting and fail at a high one. That failure often looks like an intermittent black screen.

Can a laptop drive two external monitors?

Many can, but not all, and the limit is a property of the machine rather than of the dock. If the second external screen will not appear no matter what, check the laptop's stated maximum.