After trying to upgrade to gnome 3.12 unable to upgrade// uninstalled gnome 3.12 however lost ubuntu 14.04 default login screen Plz help
You can use tty pushing ctrl + alt + f6 that goes to tty, and then you just have to type:
and that might work, and have installed it again.
This is more of a comment, but I lack the reputation... If @Michael's answer doesn't work, it could be an issue with lightdm. If so, you can install gdm to get you logged in.
To (Re)configure / switch back to lightdm