This question already has an answer here:
Since upgrading to 13.10, I can't log in to unity desktop. Light dm works correctly, but attempting to log in tries to start the session then drops back to light. I've already dropped to terminal (ctrl+alt+f2) and done this:
sudo apt-get update sudo apt-get install --reinstall ubuntu-desktop sudo apt-get install unity
Logging in as a guest session also fails. Logging in to other window managers works with varying degrees of success.
Note: I have Cinnamon 2.0 installed from PPA.
I'm using a 2 monitor setup.
Also of note is that the session prior to my upgrade to 13.10 the background of unity failed to display at all, instead showing what was there in the screen buffer from the previous frame. The entire OS worked correctly otherwise though, so I just ignored it for the session. No other upgrades or even updates were done prior to this occurring.
My upgrade path to 13.10 was basically this: Install 13.04 alongside Windows 7, use ubuntu as a glorified web browser for a while, get updates (in preparation for 13.10), install 13.10.
I also used Unity Tweak Tool to change some aspects of unity, particularly auto-hide.
Any help or ideas would be appreciated, as I'm typing this on my phone :(