Could you try to start your mac and Immediately hold the Option key. After a few seconds, the Startup Manager should appear.
If it does could you select your Hard drive then hit the right hand arrow.
Than let me know if it boots.
In that case could you Insert the Mac OS X Install or Restore disc, Restart the computer, then hold the C key during startup. From the Apple menu, choose Disk Utility. Do this in the first screen of the Installer. Don't click Continue. If you click Continue in a Mac OS X Installer version earlier than 10.2, you must restart from CD again.
Click the First Aid tab, Click Repair Disk, After repairing the disk, try to start from the Mac OS X hard disk.
Let us know what happens.