Have you tried turning it off and turning it on again? (Yes, I know just how facetious that sounds but it's amazing how often it actually works!).
If that doesn't work though, try restarting Windows Explorer. Press Ctrl, Shift and Esc together to launch the Task Manager.
It's likely that the window will hardly be populated by any tasks. If so, click on More Details.
The Processes tab will probably be pre-selected. If not, click on it.
Scroll right down to the bottom of the list, to find Windows Explorer. Right-click on it and select Restart.
If that doesn't work, please post again.