My White MacBook Screen Keeps Going To Black

For the past 2 days my less-than-2-yr-old white 13 in. MacBook has begun going to a black screen. If I really get close to the screen I can see the log-in window but cannot see my pointer. I'll hit "enter" and can faintly see the window asking if I'm sure I want to log off. Again, because I can't see the pointer, I'll hit "enter" and the computer will shut down. When I turn the computer back on it appears to be working fine. This happens while I'm running on adapter power (don't know if it also happens on battery power because I've not tried it), and I've noticed it happening only after I've left the computer unused for maybe an hour. I've tried changing the energy saver settings several ways, but nothing seems to prevent the black screen from recurring. Anyone got any suggestions? Thanks.