I have a Powermac G4 that I use as a server. Since yesterday, for any reason, it chimes, and just goes to a white screen. The only way to make it work again is to reset the PMU by holding the button inside for 10 seconds. It is running 10.4.11.
It's mostly likely a problem with the battery on the logic board, according to this document on Apple's support website. Be sure to identify the Powermac model next time though! :)