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Ubuntu does not start GUI or login loop

Always look in logs for trouble:
  • $cat ~/.xsession-errors
  • $cat /var/log/Xorg.0.log
Try manually start lightdm from command-line Ctrl + Alt + F1, log in and  

$ sudo lightdm
  1. If entering a password into display manager like gdm or lightdm and desktop is not loaded, but you get redirected back to login screen, delete file .Xauthority in your home folder and try again.

    $ sudo rm ~/.Xauthority

  2. Sometimes re-installing NVIDIA graphics card driver works, or upgrading to recent version.
  3. If you get stuck on boot screen in text mode, edit /etc/X11/default-display-manager and change it's content lightdm to /usr/sbin/lightdm.