I've installed Ubuntu 12.04 on my AMD64 desktop. The problem is that the lightdm isn't running. If I boot Ubuntu 12.04 with the Live DVD the X server runs, But If I run it from the hard drive its broken.
what is the problem?
Try reinstalling lightdm.
You mentioned, you've got acess to the terminal, so just type in:
If that was the only problem, this should do the trick.