I have a 13" macbook with 2GHz Intel Core 2 Duo, 8GB RAM running OSX 10.7.4

No problems over past 4 years but today, the screen started flickering then fade to black. I would press any key and I'd have to re-enter my password. It's happened 4 times in past 12 hours. I can't determine the cause of this problem. Your advice and assistance would be greatly appreciated.