After latest update for 14.04, cannot log in to any user account

Solution 1:

This is a known bug with lightdm. There are ways to possibly fix it, along with ways of working around the problem. Make sure your computer is connected to the internet and enter a TTY by pressing Ctrl + ALT + F2. Log in here, then try the following:

1. Purge and reinstall lightdm:
   Run sudo apt-get remove --purge lightdm. Once that's done, run sudo apt-get update and then sudo apt-get install lightdm && sudo dpkg --configure lightdm.
2. Reconfigure lightdm:
   I've never had much much luck with this, but you might.
   Run sudo dpkg-reconfigure lightdm.
3. Install another Display Manager, such as gdm:
   Run sudo apt-get install gdm.
   When you are asked which Display Manager to use by default, choose gdm.

Reboot after all of these.