First things first I guess, 1) ensure outlet is supplying power, 2) Ensure PC cord is plugged into outlet and is not damaged, 3) Ensure PC cord is plugged into PC, 4) I don't believe you have an on/off switch on the PSU but if you do, enre it is in the on position. We'll assume all is good so far and still no power on.
There are a few possibilities here, the most likely being the PSU has died but you can check that it isn't simply a broken on switch
Open the case (with power supplied to computer - but not turned on of course) are there any lights lit on the motherboard?
If yes, you will want to jump your power on pins on your motherboard
If no, you will want to unplug the cord from the PC and check to ensure all connections are firmly seated - plug in PC try again - if still no light, PSU is more than likely dead.