I had exactly the same problem and I was able to fix it by following these steps:
Shut down the Mac, then turn it on and immediately press and hold these four keys together: Option, Command, P and R. You can release the keys after about 20 seconds or so, during which the Mac might appear to restart.
If your Mac plays a startup chime sound, release the keys after the second startup sound.
If your Mac has Apple T2 Security Chip, release the keys after the Apple logo appears and disappears for the second time.
Lemme know if this fixes your issue.PS: Don't forget to take a backup of your data before you try out these steps.