Can't Figure Out What's Wrong

Here's the whole deal. The other day my computer(emachine W1800) turned on by itself and I proceded to turn it off. Then when I came next time to turn it on it failed. I figured it was the power supply that went badbecause it didn't have no juice whatsoever so I went ahead and changed the power supply but all it does is turns on the fans and the little light on the cd drive turns on but nothing else happens. I have all connected on a upc and surge protector and that day didn't have any power surges in the house. Please help, can't figure out what's wrong with it.

Thank you