If holding down the power button for longer than usual doesn't work, reset the PRAM. At one time, resetting the PRAM was the most recommended option for restoring an Apple, but judging from the literature that doesn't seem to be the case now. Nevertheless, it is a procedure that still works. Hold down the Command, Option, and R keys with your left hand, and press the P key and Power key with your right all at one time. You should hold it until you hear the start chime twice (though sometimes this doesn't happen) and give your computer a chance to reboot. It will take longer than normal because you are resetting the start-up settings to factory defaults. If resetting the PRAM doesn't work you've got serious problems that require a service visit to your local Apple store especially if your skill set does not include hardware repair. Having to resort to resetting the PRAM too many times indicates other problems, such as a need to replace your internal battery, Good Luck, Mac user since the 80's.