When you’re Mac apps misbehave, there’s only one thing to do: force quit them.
Sometimes, just like Windows programs, macOS apps can stop working of making other problems that can’t be fix with the usual means.
In this cases, you need to use the force quit option.
You can find the force quit option in main Mac menu that you open by clicking the Apple symbol in the menu bar. Alternatively, you can simultaneously press Command + Option + Esc keys.
It the force quit window you have to select the problematic app and click on the Force Quit button.
Most apps când be forced quit, with the exception of the Finder file manager, which can only be restarted, because it need to always be running.