You should boot on your OS X install CD, (put the cd in your mac, then hold C as you press the power button)
From here, first I'd do is go to disk utility and verify/repair your disk.
Doing this will likely tell you if your harddrive has crashed or is corrupt/something is wrong.
If your mac says nothing is wrong, then try "choose start up volume"
from here if your lucky there will be a start up volume named "OS X 10.*" choose that and click "restart"
if your lucky it'll work.
Truthfully i doubt that will work and you'll have to re-install OS X🙁