Press Ctrl + Alt + F2 - and you will get be able to log into the terminal. Once you are logged in, reinstall unity by doing the following:
sudo apt-get update
sudo apt-get install --reinstall ubuntu-desktop
sudo apt-get install unity
Once that is done, restart your machine. If that doesn't solve your problem then:
If you have Compiz, you might also need to reset all changes made by Compiz:
sudo apt-get install dconf-tools
dconf reset -f /org/compiz/
If you have Unity tweak tool, you might also need to do reset it:
sudo apt-get install unity-tweak-tool
unity-tweak-tool --reset-unity
If that also fails, then maybe deleting the settings files:
rm -rf ~/.compiz-1
rm -rf ~/.config/compiz-1
Lastly, you could try the following:
sudo apt-get install dconf-tools
dconf reset -f /org/compiz/
unity --reset-icons