My three year old iMac G5 was working fine all day until a few minutes ago. I was waking it up from sleep, and it worked fine for ninety seconds, but as I was reading a thread on here, the screen went blank. It was still on, but blank, and the computer was unresponsive. I had to do a hard shutdown. When I tried to poweron again, I'd hear the startup sound, but the scren never turned on, and the G5 grew from a quiet whisper to a loud roar. Resetting the PRAM didn't work. Help!