Out of the blue, my Power Mac G4 won't start up. It makes the start-up tone, goes to the silver apple screen and stays there. I tried starting up with a CD or DVD with system software (holding down 3 while starting up) and nothing different happens. I tried resetting the PRAM (holding down the command-option-P-R keys while starting up), got the second start-up tone, got the silver apple screen and it stays there. Just wondering if there's anything else I should try. It's running OS X 10.4.something - should be the latest update.