I installed Ubuntu 14.04 to replace Linux Mint Debian Edition and after installation and updating my system with this command:
sudo apt-get update; sudo apt-get upgrade
Then I could not login to my system; it would stay on the login screen. I fixed this by entering these commands.
sudo apt-get install ubuntu-2d*
sudo apt-get install gnome-session*
These commands are using the wildcard character at the end of each command to force it to find all related packages. After this I was able to log into my newly installed Ubuntu system. I hope this helps anyone else with this problem.