I upgraded my Ubuntu using sudo apt-get upgrade than after restarting a black login screen appeared. I tried to restart gdm3 by "service gdm3 restart" and than clicking Ctrl+alt+f7 nothing happened. I tried startx and some error appears. During upgrade my Nvidia GPU was say as primary GPU.
I faced the same problem, within the black login, continue to login and run command service gdm3 restart, this restarts the gdm3 service and brings back the login page. This worked out well for me using a Samsung Ultrabook i3 with Intel Ivy Bridge