I don't know what OP was running but the G5 iMacs were some of the machines suffering from the early 2000s capacitor plague[0]. The power supplies and power regulation on the logic boards would die on those all the time. If you were lucky it was just the power supply but the problem usually needed a PSU and logic board swap.

[0] https://www.cnet.com/culture/pcs-plagued-by-bad-capacitors/