Shut down the computer.

Locate the following keys on the keyboard: Command, Option, P, and R. (You will need to hold down these keys simultaneously in step 4.)

Turn on the computer.

Press and hold the Command-Option-P-R keys.

You must press this key combination before the gray screen appears.

Hold down the keys until the computer restarts and you hear the startup chime for the second time.

Release the keys.

The computer's PRAM and the NVRAM are reset to the default values. On some models, the clock settings may be reset to a default date.