Instale o desktop Unity e Unity de volta -
sudo apt-get install unity unity-common unity-services unity-lens-* unity-scope-* unity-webapps-* gnome-control-center-unity hud libunity-core-6* libunity-misc4 libunity-webapps* appmenu-gtk appmenu-gtk3 appmenu-qt* overlay-scrollbar* activity-log-manager-control-center firefox-globalmenu thunderbird-globalmenu libufe-xidgetter0 xul-ext-unity xul-ext-webaccounts webaccounts-extension-common xul-ext-websites-integration nautilus nautilus-sendto nautilus-sendto-empathy nautilus-share activity-log-manager-common python-zeitgeist rhythmbox-plugin-zeitgeist zeitgeist zeitgeist-core zeitgeist-datahub
sudo apt-get install ubuntu-desktop
e reconfigure o lightDm por -
sudo dpkg-reconfigure lightdm
depois remova o gnome usando os seguintes comandos -
sudo ppa-purge ppa:gnome3-team/gnome3-staging
sudo ppa-purge ppa:gnome3-team/gnome3-next
sudo apt-get purge gnome-shell
Espero que tenha ajudado você.