Sounds like either a corruption to Unity or a modification. Either way, following the below instructions should fix your problem.
Start by opening Terminal using (Ctrl+Alt+T) and then enter the below command.
sudo apt-get install --reinstall unity && sudo apt-get install --reinstall ubuntu-desktop
Doing this will reinstall both Unity and Ubuntu desktop essentially putting them back to default settings.
After the reinstall is complete, you will need to restart your computer for the changes to take affect.