Power Mac G4 dead in the water

Before I went to bed last night I put my Power Mac G4 to sleep, as I do every night. When I got up this morning it was dead. It doesn't respond to anything I do. I pushed on the startup buttons, wiggled all the wires, unplugged and replugged it (it was plugged into a surge protector strip and everything else plugged into the strip was running), plugged a lamp into the plug I had had the computer plugged into. It was fine. What could cause it to just die? A dead battery? Something else? I'm at a loss. I have an appointment to have the "geniouses" look at it, but if it's something as simple as a battery, that I could replace myself, I'd rather not spend the money.