I have a 17" iMac LCD running 10.2.6...I was online, and all of a sudden, the screen turned translucent grey (it was like it dimmed, but it went quickly from top to bottom) and a white box popped up that said "you must restart yoru computer, press restart or hold downthe power button" in several languages. It wasn't a kernel panic, and there was no way to close the box, or to save what i'd been working on...I've been using OS X since 10.0.2, and I've never seen this...what is it?