Reset PRAM & NVRAM on your Mac

Follow the steps below to reset PRAM/NVRAM on your Mac. 

1. Shut down your Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P and R. 

2. You can release the keys after about 20 seconds, during which your Mac might appear to restart. 

3. On Mac computers that play a startup sound, you can release the keys after the second startup sound.

On Mac computers that have the Apple T2 Security Chip, you can release the keys after the Apple logo appears and disappears for the second time. 

More information can be found here: https://support.apple.com/en-gb/HT204063