macOS Sierra: Reset your computer’s NVRAM or PRAM
If your Mac doesn’t seem to remember the speaker volume, display resolution, or the disk it should be using during startup, you may need to reset your computer’s NVRAM or PRAM.
Note: To print these instructions before you start, click the Share button in the Help window, then choose Print.
Choose Apple menu > Shut Down.
Locate the following keys on the keyboard: Option, Command, P, and R. You will need to press and hold these keys simultaneously in step 4.
Turn on your Mac.
Immediately press and hold the Option-Command-P-R keys, before the black screen with the Apple logo appears.
Continue pressing the keys until your Mac restarts, about 15 seconds.
Release the keys.
Resetting NVRAM or PRAM may change some system settings and preferences. You may need to update the speaker volume, display resolution, and startup disk preferences in System Preferences.