Yea, the idea is to access a terminal and run
so you'd let it start up as usual and then do ctrl+alt+f1 before you click your username and type in your password.
sudo apt-get install ubuntu-desktop
Like Oldfred said: Could you post back the contents of the log: /var/log/apt/history.log or attach it as a file.
Then we can see what happened last.
You could just forget all of the above and use a livecd to copy of all your data and reinstall. Actually it wouldn't be a bad idea to make a backup of important data at this stage whether you reinstall or not.