If a simple reboot of your Mac doesn’t fix things, see if the problem still happens in Safe mode (press and hold the shift key down at startup). It can take much longer to safe boot (10 min) so be patient.
Please see: Start up your Mac in safe mode - Apple Support
Safe mode is non-destructive to user data. It’s not a cure-all, but very often corrects weird software behavior. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.
Does the problem persist while in Safe mode?
Exit safe mode by restarting your Mac normally and re-evaluate the issue again.