What does it mean when the light on the front of my macbook starts flashing fast. not the normal sleep fading in and out kind of flash, it's more like a "something is severely wrong" flash. Right before it happens, my computer makes the optical drive noises as if it's going to sleep, or something, but doesn't. it seems as though it just turns itself off... I can't get it to come back, no pressing the power button, no key presses, nothing. I have to hard restart the machine by holding down the power button for a few seconds. Any ideas?