So that we can help you best, please tell us which Mac, model and year you’re having trouble with, and what macOS version you’re running. Find that info by clicking the Apple menu () at the top-left of your screen and then select About This Mac.
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 (5 min) so be patient.
How to use safe mode on your Mac - Apple Support
Safe mode is non-destructive to user data. It’s not a cure-all, but very often corrects weird software behavior by forcing the OS to do cache clearing and other housekeeping. When you're in safe boot, the machine will not be at its best performance, especially with graphics, but that's expected.
Can you empty the trash while in Safe mode?
Exit safe mode by restarting your Mac normally and re-evaluate the issue again.