Also the problem could be the switch going to your motherboard from your computer power button. if the power supply comes on, its not the problem. plug the power supply back in but don't let the paper clip lean on anything.

Take a paper clip, you may have to cut it to size, and stick one end into the green wire pin, and one in either of the black wire pins that are on the left and right of the green wire. Right beyond the connector look for the ONLY green wire that's surrounded by 2 black wires on the power supply. its the 20/24 pin depending on your computer.

Take out the big plug going to the motherboard. there is a good chance your motherboard might be dead.