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You can always check if its not something as simple as a component having come loose...

Open the machine, check if all the cards are in propperly... (you should not be able to see the golden rows of "teeth" where the card clips into the slot... so press on all the cards with MODERATE force, (please dont go snap anything)...

Check if the memory is propperly in by using the same method... and while you at it, check that the ribbon cables are in firmly aswell...

even though it doesnt boot, leave the machine on for 3- 5 minutes, and feel if any part of the computer gets excesively hot, check if all the fans are spinning. The Heatsink on your CPU should NEVER be hotter than lukewarm. for that matter no component should be much hotter than lukewarm.

If all these things are fine...

try the following:
While the machine is off
unplug the power from the unnecesary devices like CD rom, stiffy drive, 2nd hard drive (if any).
See if it will boot now...
If this help, there may be a problem with one of those devices, OR your powersupply is faulty and is no longer supplying enough stable power (and this is where my gut tells me the problem may be, if its not a faulty motherboard and/or CPU as suggested earlier).

That small fan atop the motherboard's onboard chip does not spin. I manually twirled it around (with the power off) and unplugged it and replugged it back into its power source, but it does not spin. I guess that is the problem. So now I need to practically rebuild my computer by removing and replacing the motherboard. Oh well...

The question is not, Can they reason? nor, Can they talk? but, Can they suffer?
~Jeremy Bentham