Does it show anything on the screen at all? White screen, grey screen, blue screen etc.? If the screen shows nothing, and the following instruction doesn't help anything, you may have an issue with your screen or the connection to your screen from your Mac.
Have you tried inserting the OS X disc and holding "c" while it starts up to see if it will boot from the CD? If so, you can go through the CD and use the Disk Utility to verify/repair disk permissions and verify/repair disk (if repair disk is grayed-out after verifying disk, don't worry about it). After you run both of these, reboot and see if it will load up this time.
Good luck.