Apple Users

Try this:
https://discussions.apple.com/docs/DOC-3603

PRAM RESET


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.

Why?
https://www.lifewire.com/how-to-reset-mac-pram-or-nvram-2260839

Most Mac users don't think much about their Mac's parameter RAM, but it works hard anyway, keeping track of the following:

Startup volume
Speaker volume
Display settings (resolution, color depth, refresh rate, number of displays)
Kernel panics information
DVD region settings
Date and time, including time zone
Computer name
Backlight levels
Keyboard language
Location services state (enabled or disabled)

Resetting the Parameter RAM

One easy fix for many issues is to simply reset the parameter RAM to its default state. This will cause some data to be lost, specifically the date, the time, and the startup volume selection. Luckily, you can easily correct these settings using your Mac's System Preferences.

The steps needed to reset the parameter RAM are the same, regardless of whether your Mac uses NVRAM or PRAM.

Shut down your Mac.
Turn your Mac back on.
Immediately press and hold the following keys: command+option+P+R. That's four keys: the command key, the option key, the letter P, and the letter R. You must press and hold these four keys before you see the gray screen during the startup process.
Continue to hold down the four keys. This is a long process, during which your Mac will restart on its own.
Finally, when you hear the second startup chime, you can release the keys.
Your Mac will finish the startup process.