You could try to rename all directories containing configuration files and cache in your home directory - they should be newly generated after logging in again. I am quite sure that this will work, but ALL YOUR SETTINGS WILL BE LOST.
First, switch to the command line interface using ctrl+alt+F1
and log in.
Then rename all directories containing configuration files (do NOT delete them, just in case) using the following commands
mv .config .config_old
mv .cache .cache_old
mv .gconf .gconf.old
After having done this you have to log out and back in. As this is not possible using the GUI you can run the following command:
sudo service lightdm restart
This will restart the Xserver and you should be able to login. Unity should now work again, but all your settings are reset to their default values.