Does not turn on ...Can you help?

Tough one -- could be just about anything. But the last time I had a computer similar to this conk out on me (it was a B&W G3), I checked all the hard drive connections, unplugging them and replugging them. If I'm remembering correctly, I think I also swapped the PRAM battery with a PRAM battery from a different, working computer. And I may have removed and reseated the RAM. The computer eventually started back up, but I'm not certain exactly which of my various tricks brought it back.

UPDATE: You can find teardown and repair guides for some G4 PowerMacs here: Power Mac

A couple of safety tips, in case you haven't opened a machine like this before -- be sure to unplug the machine before you open it up. Be sure to wear an anti-static strip and/or frequently touch metal while working on the machine to decrease the chance of static electricity frying the machine. And take special care when working inside these machines -- there are some sharp metal edges inside and you don't want to cut yourself.