Computer won't turn on

[Dawnstar]

So I got home from school, tried to turn on my computer but realised the power was down. So about 3 hours later, power is back. I try turn my PC on but it won't turn on, I just get really quick high pitched beeping noises coming from the power unit.

Tl;Dr Computer won't turn on, weird high pitched beeping noise coming from power unit.

 

[Cycloid Torus]

Power Unit? Power Supply Unit inside computer system case? See #2

Power Unit? Battery Powered Uninterruptable Supply? See #1

#1. High pitched sound from UPS - likely normal: indicating power was off, possibly needs re-charging, possibly needs reset.

#2. High pitched sound from Power Supply Unit - likely bad PSU - do NOT open; too dangerous unless you have technical training - borrow a known good PSU and see if it works.

If you are not technically inclined, ask for help.

 

[Dawnstar]

How so dangerous?

 

[Cycloid Torus]

Enough voltage stored in capacitors to blow you across the room and (probably) kill you.