GDM = GNOME Display Manager
You may need to restart GDM if GDM halts or does not want to function anymore.
If a similar problem like this occurs it usually means that only GDM does not function.
If you quickly want to restart GDM press the following key combinations together:
To restart GDM via the Terminal session type in the following command:
sudo /etc/init.d/gdm restart
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