if you are a gnome-3 user like me, you might also be experiencing some troubles like "failed to laod session gnome-3" when you update with the gnome-3 ppa. the laziest method is to sit back and wait for some new updates from gnome-3 or you can purge gnome-shell (purge means removing all the files created by the it) and then install it again. execute these .
sudo apt-get update && sudo apt-get upgrade // to check for some last moment updates
sudo purge gnome-shell
sudo install gnome-shell
