Well, if you're MacBook ain't mooing chances are it could be shutting down on you. Apple fessed up today saying that a small number of lazy ass MacBooks have been intermittently shutting down on their owners. A new firmware update should keep your sleepy Mac from quitting on you. Or you could give it the ol' German technique.
DISCUSSION
If this update doesn't fix that problem for you, definitely take your MacBook into the Apple store as it could be a hardware issue...brilliant. wisdom thrown down from the mountain that is trendspotter. some more useful advice: if your computer doesn't work it might be broken.