if your 100% sure its not a power supply issue, and the computer is kept clean with antivirus etc. and you have done a bios re-set, and checked the bios battery is not dead...
my guess would be that your motherboard has "danced its final dance"

a pain in the ass but it could be worse
babystarbud Reviewed by babystarbud on . Dammit, my computer broke! One minute it was totally fine, next thing it just turned off all the sudden, and can't turn it back on. Tried switching power sources, and power cables to no avail! What happened?!?!! Help me! I have so much shit saved up and I can't lose it! I backed some stuff up, but not everything! Now I'm stuck with a stinkin' laptop! Rating: 5