I had that too. It seems that 14.04 uses something called ubuntu-session for login, while the old 12.04 used something called unity-desktop. Since I tried to upgrade from the desktop itself, there were problems deleting it, and I was barred from logging in the next time.
To fix, boot the machine, and as soon as the grub loader comes up, scroll down to "Advanced options for Ubuntu" which will probably be the 2nd entry. Hit enter. On the next page, choose "Ubuntu, with Linux 3.13.0-30 (recovery mode)" or a similar entry. It could be the 2nd entry on that page. This brings up some recovery options.
Scroll down to the thing that says "network Enable networking" and hit enter. This is IMPORTANT, otherwise Ubuntu can't find the files it needs on the internet. After networking is enabled, scroll to "dpkg Repair broken packages" and hit enter. That should install the needed ubuntu-session, delete the old unity-desktop, and fix up any other things that were missed also.