Computer won't turn on, or the screen stays black
Most computers that seem dead aren't. They're out of power, stuck, or showing nothing on a screen that is actually on.
What's usually going on
The usual causes are a charger or cable that has failed, a battery drained so far that it needs time before it responds, or a computer that is running but stuck with a dark screen. A truly failed computer is the least likely explanation.
Worth trying first
Check the power, properly.
Try a different wall outlet and, if you can, a different charger or cable. On a laptop, leave it plugged in for thirty minutes before trying again.
Force a restart.
Hold the power button down for a full ten seconds, let go, wait a few seconds, then press it once.
Rule out the screen.
Turn the brightness up. Listen for a fan or a start-up sound. If you use a separate monitor, check its power and cable, and that it's set to the right input.
Unplug everything else.
Disconnect drives, printers, hubs, and anything else attached, then try again. A faulty accessory can stop a computer from starting.
When to hand it over
If it still won't start, stop there, especially if what's on it isn't backed up. Repeated attempts and repair-shop resets can make files harder to get back. We'll work out whether it's the charger, the screen, or the computer itself, and get your files off it first where that's possible.
Your own technologist, on call.
Tell us a little about your setup. We'll take it from there.