- Shut down your Mac.
- Locate the following keys on the keyboard: Command (⌘), Option, P, and R.
- Turn on your Mac.
- (This step requires some dexterity!) Press and hold the Command-Option-P-R keys immediately after you hear the startup sound.
- Continue to hold the keys down until you hear the startup sound twice.
- After the second chime, go ahead and release the keys.
After resetting NVRAM, you may need to reconfigure settings for speaker volume, screen resolution, startup disk selection, and time zone information.