Here are some things to try:
1) Try to fix the disk from single user mode, if you don't have boot discs.
During boot, hold Apple-S
You should get a text-based interface with a prompt.
At the prompt, type this exactly:
fsck -fy
(and press return)
You should get a bunch of text that may or may not end with a phrase similar to "repair completed successfully." If you get this, you can type reboot to restart the Mac. If you get something that indicates that this failed, then come back with the message you got.
2) Reset non-volatile memory --
see these instructions
3) Reset Power Management Unit --
see these instructions
Try #1 first.