My macbook had been working fine. I was online and stepped away for a few miutes. The machine went to sleep.
Then, it suddenly started making three loud beeps in quick succession -- like some kind of alarm -- followed by five seconds of silence, then three loud beeps. It kept doing this cycle after cycle and the screen remained black. The power light on the front panel would blink in time with the beeps but otherwise would be dark.
The beeping stopped when I held the power button for five seconds. When I press the power button again, the beeping alarm starts again and the screen remains black.
The three beeps is due to a RAM problem - try re-seating them, if this doesn't help take it back to the store or if out of warranty just buy some more (and make it bigger - you can never have enough!) Good luck.