Go to System Settings, then Display Configuration, then Compositor and see if there's a message there about Compositor having crashed and you have to click on something to re-enable it. If it doesn't get re-enabled, try making a change to "Allow applications to block compositing" and Apply. If this doesn't fix it, try pressing Ctrl+Alt+F12 but be advised [!] that it can crash the system (it crashes mine) and you have to Ctrl+Alt+Del then to restart.
But again, this should work:
1. System Settings
2. Display Configuration
3. Compositor
4. See the warning and respond to it there.