It might be that the hard drive has given up. Or a prob with RAM. I had a similar problem and managed to change the HDD myself without too
much trouble.
Only thing is, for some reason I can't install software updates.🙁
still the machine works ok & the G4PB still looks great today, even next to new Macbook pros.
alternatively, reboot and hold down shift (safe[er]-boot with some things disabled), or reboot and hold down cmd-v (verbose boot mode... you can read the text and see what it tells you or where it hangs).
However, booting from the cd would be your best bet.
Yes, def try the install disc first to see if there is a problem with HDD first!
I heard clicking noises before my PB gave up, when I tried to reboot it would just get stuck on a blue screen with a questionmark/finder logo in the middle. If it is the hard drive, theres a website that shows very clearly how to change it yourself.
Disc utility will tell you if there are problems with your HDD and whether it can fix those problems. If the HDD is completely kaput then disc utility will not be able to locate it.
so I did Archive Install and after the reboot its stuck on the apple logo with the wheel on the gray screen. It's been a good hour stuck here. suggestions?