To reset your computer's PRAM:
- Shut down the computer.
- Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
- Turn on the computer.
- Immediately press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Continue holding the keys down until the computer restarts, and you hear the startup sound for the second time.
- Release the keys.
Resetting PRAM may change some system settings and preferences. Use System Preferences to restore your settings.