Some suggestions in the order I would try…
- Restart Cubase
- Restart your computer
- Start Cubase in safe start mode
- If on a PC (not sure about Mac) try to “Repair” Cubase from the windows apps & features or programs & features menu (you might have a “Repair” button or you might have to hit “Uninstall” to get the repair function).
- Reinstall Cubase
Just guessing as you did not list your Cubase version or your computer OS. 
Regards 