Thank you for the last part of last answer:
Similar issue on 14.04 where "gdm" is again broken/no longer supported?
I was in a bind after reboot *I could no longer login to Ubuntu: NO login screen -- only a background and mouse (nothing else).
Then recalled I had selected "gdm" as default for lightdm reconfiguration once (as part of a successful package installation).
Login from a terminal (Ctrl+Shift+F1)
and then enter:
sudo dpkg-reconfigure lightdm
Then selecting default "lightdm" from the TTY menu choice.
I was able to restore a working login screen (and it still worked across reboot).
Basically after 12.04, never select anything but default "lightdm"...
(Unless you have installed something else that you know for sure will work -- e.g. KDE)