After upgrading my installation of 12.10 Quantal Quetzal to Ubuntu 13.04 raring ringtail Unity decided to go in to hiding mode. Not sure why this happened, but resetting Unity And Compiz solved the issue for me.
Resetting Unity and Compiz is exactly what it sounds like. It will reset all customizations and configurations you’ve done to Unity and Compiz. So make sure that you backup any settings you wish to keep. Your mileage may vary.
Before resetting we need to install dconf-tools on your machine:
sudo apt-get install dconf-tools
Now run the following command to reset Unity and Compiz:
dconf reset -f /org/compiz/
All you need to do now is to logout and login and Unity should re-appear on you brand spanking fresh version of Ubuntu. Enjoy!