If you have access to a FireWire cable and another Mac, consider booting your MacBook in target disk mode. IF your computer responds and boots, you can then copy files off your machine and also attempt a permissions repair (not that I think this will cure your problem.
For this discussion, your problematic MacBook is Computer #2 and the other Mac is Computer #1.
- Boot Computer #1.
- Plug a FireWire cable into #1.
- Connect Computer #2 to power.
- Connect the FireWire cable to #2.
- Boot #2, holding down the "T" key until the FireWire logo appears.
At this point you should be able to see Computer #2 in the Finder on Computer #1. It will act as a regular hard drive as far as Computer #1 is concerned.
Retrieve your needed files, use Disk Utility to Repair the disk and Restore permissions. YOu can even check it with a program like Drive Genius for errors.
If the MacBook boots to Target disk mode, you know it's not the motherboard or RAM. At that point, I'd reboot into Repair mode by holding down COMAND-R at boot. From here, you can reformat the drive with Disk Utility and reinstall OS X.