Have you installed Gnome 3 from ppa:gnome3-team/gnome3 because you were tired of Unity or just because you wanted to test the new shell and this has broken your login? Are you now on the login screen but no graphic environment will ever work?

Then you have to remove gnome 3 following the steps below:

–         Press CTRL+ALT+F1 to access the terminal window and log in. At the prompt type following commands:

–         sudo apt-get remove libgtk-3-common

–         sudo apt-get install ppa-purge

–         sudo ppa-purge ppa:gnome3-team/gnome3

–         sudo apt-get dist-upgrade

–         sudo apt-get install gnome-panel

–         sudo apt-get install lightdm

–         sudo shutdown –r now

The pc will reboot and you should be able to use Unity/Classic login again.

05/04/2013 *UPDATE* (thank you Anthony)!:

If after the reboot the machine is not booting in Unity or Gnome3 and it’s just loading the terminal, maybe you’ll have to reconfigure lightdm to be the default login manager:

–         sudo dpkg-reconfigure gdm

You will see a window similar to this one (where you should then choose “lightdm”)

Good luck!