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1. Shut down the computer. 2. Unplug the computer's power cord. 3. Wait fifteen seconds. 4. Attach the computer's power cord. 5. Wait five seconds, then 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.