I recently had this problem, but somehow resolved it. I get to the gray screen with the logo, in normal startup, and in safe mode I get the addition of a spinning gear, but it never finishes booting. It had been without power for too long and the CMOS battery had died. I replaced it with an old battery that had a full charge. Thru some weird order of steps, I got it to run again, but since now it won’t boot.?

It is a G4 2002 Quicksilver, overclocked and dual CPU. It had previously run stably for 10 years.

Can someone here make some suggestions, or suggest a list that might help?