Our G4 was similarly not budging, EXCEPT there was just the hint of a tick sound, every second or two.. Can you hear any sound whatsoever?
With ours, we did the scary-anti-lawyer step of opening the box, with strongwilled intention to not touch anything, just look, and sure enough when we plugged in the G4 and watched, you could see a tiny flash every second or so that corresponded with the ticking. Unplug electric cord because answer is clear. It's not the power supply as many thought, nor the pram, etc.
The good news is that seeing is useful. The not-so-wonderful news was that the flashing was an obvious sign of a fried logic board, which we then found a replacement online and see the replacement steps here at ifixit.
best wishes... ttyl