I just updated Ubuntu about 15 minutes ago on my desktop which is running 12.04. Within the set of updates, there were some kernel upgrades. It didn't say I needed to reboot after the update was finished, but I thought I would anyway.
The computer successfully rebooted up to the login screen. However, after entering my password, the screen goes black for a moment, then it comes back to the login screen.
I tried the suggestions in this question, but unfortunately they didn't help. In the accepted answer, the questioner vaguely says it was a "Unity problem". I'm using Gnome-classic with full Compiz effects. I tried uninstalling and reinstalling Compiz, but that didn't fix anything.
So... I can't use my desktop computer until this is fixed. I hope there is some help soon.
I've uploaded the output of
cat /var/log/Xorg.0.log to Pastebin.
A temporary solution of changing to gdm instead of lightdm was found with help in the chat room.