I had a G4 iMac that suddenly "died" a few years back. Unplug it, put your ear up to the top vent, plug it back in and try to turn it on. If you hear a faint clicking sound, it has a dead power supply. I found a second hand one on Ebay and it was fine from that day on out, but keep in mind that the power supply is at the very top of the iMac, so replacing it is a time consuming, very tedious process. Not for the faint of heart that's for sure.