Type ctrl+alt+[a number in (1-6)] in order to enter in text mode.
Open your user sesion (with your username an password).
Type:
and
in order to stop and restart your gdm3 server.
Open your user sesion (with your username an password).
Type:
sudo /etc/init.d/gdm3 stop
and
sudo /etc/init.d/gdm3 start
in order to stop and restart your gdm3 server.
