1.Shut down the computer.
2.Plug in the power adapter.
3.On the built-in keyboard, press the Shift, Control, and Option keys on the left side and the power button, all at the same time.
4.Release all three keys at the same time.
5.Turn on the computer.

1.Shut down the computer.
2.Press the power button.
3.Before the grey screen appears, press the Command, Option, P, and R keys at the same time.
4.Hold the keys until your computer restarts and you hear the startup sound a second time.
5.Release the keys.