Try this: 1. Shut down the computer. 2. Plug in the power adapter to a power source, connecting it to the Mac if its not already connected. 3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time, if it starts up you held the power button before the other keys, try again. 4. Release all the keys and the power button at the same time. 5. Press the power button to turn on the computer. 6. Press and hold the Command-Option-P-R keys before the gray screen appears. 7. Hold the keys down until the computer restarts again at least twice. 8. Release the keys.