Cannot pass the login screen after upgrade to 12.04 from 11.10

I had the same problem. This is how I solved it (parts only in German, sorry; try e.g. Google Translate):

1. chroot: http://wiki.ubuntuusers.de/chroot/Live-CD

2. Look for the post of bowman Mai 2011 12:52: http://forum.ubuntuusers.de/topic/daten-retten-mit-live-cd-2/#post-2922047

3. Run the following commands in a Terminal:

   sudo apt-get clean
   sudo apt-get update
   sudo dpkg --configure -a
   sudo apt-get -f install
   sudo apt-get upgrade
   sudo apt-get dist-upgrade
   sudo apt-get install --reinstall ubuntu-desktop
   

This did the trick.

Perhaps there's a problem with your desktop environment (or LightDM). IDK which desktop you are running (Perhaps you should add this to your question), so I'm going to assume your running the default one, Unity (aka ubuntu-desktop).

I would try reinstalling both the desktop and LightDM. You can do this by entering a TTY terminal (CTRL+alt+F1 for TTY1, CTRL+alt+F2 for TTY2, etc. CTRL+alt+F7 to return to your desktop). Login, and run:

sudo apt-get clean
sudo apt-get update
sudo apt-get install lightdm ubuntu-desktop --reinstall

From there, I would reboot, and see if that fixes the problem.

If you're using some other desktop, you should be able to replace ubuntu-desktop with the name of your prefered desktop.