My desktop pc went dead on me while I was using it.It rebooted itself a couple of times on its own before turning off and going dead.I was told it was probably a power supply problem and that I should replace the power supply.I changed the power supply and the computer is still dead.A local tech gave me a used power supply to try and get my computer running again so I can start transfering the data to another pc.When I installed the power supply the green light on the back was lit up.When I plug the 24 pin plug(P-1) into the motherboard the green light on the back of the power supply goes out.The fans do not run either.Also,before my computer died,the dvd/cd tray would open and close by itself.What could the trouble be?