If you get a screen with a dialog box that looks like this (the specific wording changes from version to version of macOS), reset the PRAM
How to reset PRAM
Reset PRAM
- Locate the following keys on the keyboard: Option, Command (⌘), P, and R. You will need to hold these keys down simultaneously in step 3.
- Turn on your Mac.
- Immediately press and hold the Option-Command-P-R keys. You must press this key combination before the gray screen appears.
- Continue holding the keys down until your Mac restarts, and you hear the startup sound for the second time.
- Release the keys.