My laptop was working perfectly yesterday, and now today I can't log in.

At first, I was only getting a black screen. However, after following instructions in some questions here on AskUbuntu, I first reinstalled lightdm, and then I had to change the ownership of the file ~/.Xauthority to be my user name. Now, I get the log in GUI screen.

However, once I enter my username, it flickers and then comes back to the login GUI. It does not matter if I use Gnome, Unity, or Gnome-Flashback.

I don't know why lightdm needed to be reconfigured, or how ~/.Xauthority got changed, but in any case, what is still standing in my way, preventing me from logging in?

Update: I have tried deleting the .Xauthority file, and the .profile files in my home directory. It has not changed anything.

Logging in as guest also fails to work.

The following commands did not work:

mv ~/.config ~/.config.BAK
mv ~/.cache ~/.cache.BAK

Inside .xsession-errors in my home directory, it says:

Gdk-CRITICAL: gdk_x11_display_get_xdisplay: assertion 'GDK_IS_DISPLAY (display)' failed