A couple of things to try,
Troubleshooting: My computer won't turn on and
Resetting your Mac's PRAM and NVRAM. Look for your install DVD set, pop it into your optical drive and restart holding down the
D key and when prompted, ask for the Extended Test.
Also if you have MainMenu or OnyX, run all the cleaning cache, rebuilding and maintenance tasks, restart, see where that takes you and report back.
If those don't fix anything, can you recall what you did last before this happened? Any new peripheral connected to your Mac? Any new software you installed right before? Any new setting in your preferences?