What do I do when a computer won't turn on?

Three checks, in order:

• Is anything happening at all? Lights, fans, any noise. Nothing at all points at power: the cable, the outlet, the power strip, or the power supply. Try a different outlet directly rather than through a strip.
• Does it power on but show nothing? That is usually the monitor, the cable, or the input source rather than the computer. Check the monitor is on, check the cable at both ends, and check the monitor is on the right input.
• Hold the power button for ten seconds to force it fully off, wait, then start it again. On laptops, unplug and, if the battery is removable, take it out first. This clears a surprising number of stuck states.

What not to do: if there is any burning smell, or the machine is making a repeated clicking or grinding noise, stop and unplug it. Running it further can turn a recoverable situation into a data recovery job.

If there is work on that machine that exists nowhere else, say so when you call. It changes what we do first, because the priority becomes getting the data off before anything else is attempted.