Okay there could be four culprits...
1)You need to zap the P battery...At the back of the motherboard around the PCI slots, there is a tiny black button... You hold it down for 10 seconds...Then you try to turn your computer on...If that doesn't work, this is where it gets hairy...
2)Your motherboard is FRIED. You will need a replacement, and may god have mercy on your poor soul if you don't have applecare...
3)Your power supply is dead, this is also pretty bad, but not as bad as 2...
4)Your processor is dead, this is pretty bad, but this gives you an oppurtunity to upgrade to something with a little more VROOM VROOM
Well hope this helped,
-void