The computer sounds like it is running but the screen is black, is it broken?

Usually not. If the fan spins and the lights are on, the computer is running and the picture is either going somewhere else or the screen is dimmed. Raise the brightness and press the Windows key and P to choose where the picture should appear.

What does the Windows key plus P do?

It opens a small menu where you choose where the picture goes: only the computer screen, only the external one, the same picture on both, or extended across two. If that choice lands wrong, one screen looks black even though everything works.

The screen is black but I can hear Windows starting, what now?

Unplug every external screen and restart. If the picture is still assigned to a screen that is not connected, the built-in one looks empty. Otherwise hold the power button for ten seconds, wait a moment and start the computer again.

How do I know whether it is the cable?

Swap in a different HDMI or DisplayPort cable, ideally one you know works, and try another socket on the screen. Cables wear out where they bend, and a broken one gives exactly the same black screen as a larger fault.

Black screen? How to get the picture back on your computer

A black screen usually means the picture is being sent somewhere else, not that the computer has broken. Listen first: if the fan spins and the lights are on, the computer is running and the question is where the picture ends up. The first steps take under five minutes.

Try this first

Wake the computer properly. Press a key and move the mouse rather than just opening the lid. The screen should light up within a few seconds.
Raise the brightness. On a laptop the keys sit along the top of the keyboard and carry a sun symbol. On some models you hold the Fn key at the same time.
Unplug external screens. If a monitor, a TV or a docking station is connected, disconnect it and restart the computer. The picture may be sitting on that screen.
On Windows: press the Windows key and P together, then use the arrow keys and Enter to choose PC screen only. You cannot see the menu on a black screen, but the choice still registers.
Shut down fully and start again. Hold the power button for ten seconds until the computer goes quiet, wait half a minute and press it again.

If that did not help

Check that the external screen is switched on and that the right input is selected in the screen's own menu, for example HDMI 1 or HDMI 2. The buttons are usually underneath or behind the screen.
Reseat the cable at both ends, and try a different cable if you have one. Cables fail more often than people expect, especially where they bend behind a desk.
On Windows: Settings > System > Display > Multiple displays, then press Detect.
On a Mac: System Settings > Displays. Hold the Option key and the Detect Displays button appears.
Connect the screen straight into the computer. If you use a docking station or a USB-C adapter, take it out of the chain for a moment. Adapters do not always handle the resolution the screen wants.
On a desktop computer: make sure the screen cable goes into the graphics card's socket, low down on the back, rather than the motherboard's socket higher up.

If the computer starts with sound but never shows a picture on any screen, or beeps a few times at startup, something needs looking at physically. Ask us first and you will know what is worth taking in for repair and what you can settle yourself.

Common causes

The wrong input selected on the screen is the most common of all. A screen with two or three sockets stays on whichever one was used last, and shows an empty picture until you change it in the screen's own menu. Next comes a broken or overly long HDMI cable, and after that a docking station that cannot drive two screens at once. On laptops, brightness turned all the way down is also more common than people expect, particularly if someone else has used the computer or it has been asleep for a long while.

If the picture comes and goes, or flickers for a few seconds before disappearing, the cable is the first thing to replace. A cable pinched behind a desk or bent sharply loses contact at the slightest movement. Move it gently while the computer is running: if the picture blinks as you do, you have your answer, and a new cable is cheap and settles it for good.