Start with the right checks.
A spinning pointer in one application does not necessarily mean the whole Mac needs to be forced off. Check which application is actually stuck.
What to do next.
Wait briefly if the application is processing a large file, and look for a hidden dialog.
Try Command + Q or the application’s Quit command.
If that fails, press Option + Command + Esc. Select only the unresponsive application and choose Force Quit if you accept losing its unsaved changes.
Reopen the application and check for its own recovered document or recent file.
Check the result.
Repeat the original task with a noncritical file. Note whether one particular document triggers the failure.
Stop and get help if…
The application repeatedly freezes, several applications fail together or the problem involves business data shared with other people. Avoid treating repeated Force Quit as a lasting fix.