Ubuntu 12.04 Log in screen freezes after password

Try to log in unity 2d and disable the cyclic wallpaper (if enabled). This solved for me.

The bug is been fixed in
https://bugs.launchpad.net/ubuntu/precise/+source/unity-greeter/+bug/986967

Unity 2D has the same problem. I don't have cyclic wallpaper activated.

How did you get 12.04? If an upgrade, how did you do the upgrade? Are you dual booting? If so, is Ubuntu a wubi installation? If it was an upgrade, please burn a live-CD and see if 12.04 will run directly from the live CD.

Maybe try workaround #4 (rename or rm ~/.Xauthority) and reboot.
https://bugs.launchpad.net/unity-greeter/+bug/986967/comments/4

Anyway, I'd work from tty: ctrl+alt+f1
sudo stop lightdm
sudo dpkg-reconfigure lightdm
sudo restart lightdm
or
sudo service restart lightdm
or
sudo reboot

Verify if lightdm is default dm:
nano /etc/X11/default-display-manager

Content should be:
/usr/sbin/lightdm

Basically clean-up and update to exclude broken/missing package story.
apt-get clean
apt-get autoremove
apt-get update
apt-get upgrade
apt-get -f install

It's 12.04 amd64 installed from live-CD with no dual boot, sda1 ext4 /, sda3 ext4 /Home, sda5 swap. It runs fine from live-CD. I also tried to re-install and update the first time the problem appeared, that solved it for a little while. It seems to appear after several successful boots.

Got as far as nano in #4, I'll try the rest when I have net access.

I tried all of #4. It didn't work.

Just a thought ... I've seen a few comments in which there was a problem of some kind, and each of these had a separate /home partition. Just thinnking out loud .... I wonder .... ?