Solution:

Turn off the Mac.

Found the following key on the keyboard: Command (? ), options, p and R.

Turn on the Apple computer.

Press and hold the Command-Option-P-R key immediately after hearing the startup sound.

Press and hold these keys until the computer restarts, and then you will hear the startup sound again.

Release these buttons.

After resetting NVRAM, you may need to reconfigure speaker volume, screen resolution, startup disk selection and time zone information settings.