Resetting PRAM and NVRAM
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.
Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
Hold the keys down until the computer restarts and you hear the startup sound for the second time.
Release the keys.
Your computer's PRAM and the NVRAM are reset to the default values. The clock settings may be reset to a default date on some models.
n some troubleshooting situations, if resetting PRAM does not resolve an issue, resetting the PMU may be the next appropriate step.
Force Quit (Option-Command-Escape)
Force Shut Down (press the power button for 10 seconds)
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