Looks like GDM is not even starting so I am guessing after the error you are getting to a terminal login instead of the GUI login? If that is the case and assuming there are no other problems you should be able to login at the terminal and then type
This will get you into GNOME the GUI interface. So you can work in a more familiar environment. After getting into the gui I would suggest using Synaptic to remove and then reinstall GDM. You can get to Synaptic by clicking on System/Administration/Synaptic Package Manager.
Alternatively you can remove and reinstall GDM with apt-get from the terminal.
sudo apt-get remove gdm
You can also use apt-get reinstall but I alway like to remove the troubling package that I want to reinstall.
After if finishes removing the package
sudo apt-get install gdm
This may and hopefully will solve your problem. I have never had this problem before so I am just making a guess as to what the simple solution may be.
As a word of advice for post to LQ. Use a more descriptive subject then Minor Problem. Often people look at threads based on there knowledge of the subject and so someone who could offer help may pass over the non descriptive subject. A good subject for this thread would have been 'GDM errors when starting Ubuntu'.