|Sounds like the hard drive has failed. The PowerBook G4s are NOT that easy to disassemble, but I suppose you could try fixing the problem.|
Have you tried booting it from a CD/DVD, such as a Mac OS X install disc, the Apple Hardware Test disc, or one of the other discs which you should have? To boot from a disc, insert it and hold down the 'C' key. Then reboot the computer, and keep on holding the C key. Keep on holding until it recognizes the disc and starts booting successfully.
What model of PowerBook G4 is it? Titanium? Aluminum? Display size? CPU speed?