My whole G4 Mac actually froze up on me today. It had me very confused, as it had never done that before. I got in sort-of through ssh from another computer, but it didn't make it as far as the prompt. So, I cold booted, and... nothing. Left it off for a bit the next time, finally got the Apple logo to come up, but it just stayed there.
Next I will try to find my Tiger cd... or the original 10.3 cd, and see if that will boot. Bah.
Edit: Typing this from the Mac. Whew, after several tries got the Tiger CD to boot, checked the hard drive, fixed the permissions, and then got it to boot from the hard drive again.