.....your screen goes gray and a darker gray massage box comes up saying 'mac OS X needs to restart' What is that? is that a kernal panic? it happened few days ago and and i had no idea what was going on!
yes thats a kernel panic. if it happens regularly then something is wrong with your setup. (firstly disconnect all unnecessary usb devices, and try a basic config for a while, if that doesn't help, it might be a software issue.)
i've had about one KP in a year on my macbook. you shouldn't see this very often!
As vandozza said, kernel panics should be extremely rare, if they happen at all. Of the three Macs I've owned since 2007, and the half dozen or so at work I also use from time to time, I've only had one kernel panic, and that was because I mistakenly unplugged a USB hub with devices plugged in, in the middle of a file copy.