As you probably broke a package related to the operation of the Desktop Environment:
You should install another Desktop Environment for example
lubuntu-desktop and after that remove
gnome, then reinstall it again, if everything goes ok remove lubuntu (if you want).
To install Lubuntu(LXDE for Ubuntu) (as said on this answer on AskUbuntu) type the following on the command-line:
As you are having problems to use the Desktop Environment, push the following buttons Ctrl + Alt + F1 to go to the tty1 and login again - to come back to the another interface push the following buttons Ctrl + Alt + F7 - after you are logged in tty1 enter with the following line of code:
$ sudo apt-get update && sudo apt-get install lubuntu-desktop
$ sudo apt-get remove unity gnome
After that come back to the graphical interface and reboot the system or type:
$ sudo reboot
Then if everything is ok after booting the system again, reinstall gnome and unity again:
$ apt-get install unity gnome
And reboot the system again.
That's it, I hope I have helped you.