I have a 08 Mac Pro running 10.6 and while I was using it, it suddenly shut down. I tried to press the power button on. I can hear the click sound but just after 1-2 seconds, another click sound occurs and the power light is off... So I even couldn't see any grey screen nor the apple logo... Seems like it shuts down again by its own... Does anyone have any idea what has happened?? It frequently shut down on its own previously. I repaired the permission and went back to normal now, it shut down completely and won't allow to boot up the system at all... Many thanks!!