Can you boot to single-user mode? Restart, holding Command-S.
Try running fsck -fy at the command prompt.
If you have OS X 10.7 or higher, try booting to your recovery system. (Restart, holding command + R)
If you can get to the menus, try checking your hard drive in Disk Utility.
If you also can't boot to the Recovery system, then it is possible that your hard drive is failing, or already failed.