You could try booting your computer with the Option, Cmd, O (letter o, that is) and F keys all held down while it chimes the boot. This should boot into Open Firmware, in which it presents you with a command line prompt. If you can see the prompt then your hardware is probably OK, and it is a software issue. If you can't see the prompt then your hardware is likely hosed; sometimes people can still use their computer when this happens by connecting an external display before boot, and leaving the lid down.
If it is a software issue then you may need to reinstall the operating system from the restore disk.
Kurt