Is it your ibook is in sleep mode? If you leave it alone for about 20 minutes then yes, the screen will go black and you have to move the mouse to make it "wake up." I doubt if this is your problem though. Try running Pixel Tester to see if you have any dead pixels on your computer. Is your ibook still under warrenty? If yes, then if it keeps happening and its not sleeping, take it into apple yourself and show them what is wrong and see what they even do to it. If you have any dead pixels, apple will replace the ibook for free if its still under warrenty. And when the screen goes black, does the machine shut off or just the screen? Can you hear the hard drive running at all or is it totally dead? What do you do to get the screen back to normal? More info would help.