Try the following:
1. Boot up holding down the option key. Can you see your internal drive as an option? How about the startup disk that came with your machine? If you have an external bootable drive connect that, can you see it? If you don't see any bootable systems while booting with the option key being held dow, then you've got a serious hardware problem.
2. Zap the PRAM by holding down the Option-Command-"P" and "R" keys- Probably won't help but you never know.
3. Can you boot up in target disk mode. Hold down the "T" key while booting up. You'll end up with a FireWire icon on the screen. If not serious hardware issue again.
4. Schedule a genius appointment at an Apple Store and get a quick evaluation by an expert.