My G5 Tiger Mac would not wake from sleep (and could not be manually put to sleep) after a power surge (lightening storm). I fixed it temporarily by verifying and repairing disk permissions, but than it happened again (despite being on a surge protector). I spent many hours trying to figure out a solution to no avail and finally my spouse physically took it in to a shop where it was diagnosed as having several blown capacitors on the power supply.

I just wanted to bring this possibility up so that others may not waste quite as much time as I did on "keyboard" fixes. The repair shop (ACS []) replaced the blown capacitors and it was relatively cheap (many thanks ACS). The other avenue of repair would be to replace the entire power supply unit. If anyone is interested I can forward a picture of what my blown capacitors looked like (bulged out and/or corroded) upon request.