I know this is an ancient thread, but just in case anybody is trying to resurrect their old flat panel G4, there’s another trick to try before you crack open the case: use a hairdryer to blow warm air into the fan opening at the top and the air ventilation around the bottom for a few minutes, then try powering it on again. I’ve used this trick to restart my own iMac when it showed no signs of life, and it has worked for me twice. 21 years old, and still runs!