Odds are the PRAM (a.k.a. backup) battery has expired and will have to be replaced before you will be able to reboot. That happened with some regularity on PPC Macs. The battery generally runs around $15 to $20
Hopefully, you're right. A friend wasn't so lucky. Not being technologically savvy in how to do a forced shutdown, he pulled the plug a few times on his (formerly my) Gigabit Ethernet G4. He reset PMU and replaced the battery to no avail. My best guess is that the power supply failed but I'll never know because he replaced it with (horrors!) a Windows box, which he hates, by the way.
