Doubt it's a stuck power switch? Cause when it does power on it should recognize a OS, it's not even getting there. Possibly a hard drive gone bad? Are you able to get into the BIOS or boot from the OS CD disk? The hard drive should have a smooth spinning sound when running. See if the hard drive is making unusual sounds, here's a link that may be of some help.
datacent.com/hard_drive_sounds.php
Also is the power supply fan running when the computer is powered up? Possible bad power supply, but when they go, alot of the time the computer is completely dead. Or if the power supply feels hot to the touch that is a bad indication.
You can try and pull all the ram out and then power up the computer and shut it down. Then add one ram module back at a time as you power the computer on and off till you have them all back in place.