I have already researched solutions to fix this: After logging in, there's a black screen and my cursor, nothing else! in Ubuntu 12.10 But I have more questions that need to be answered. Knowing how anal the interwebz gets about duplicate posts, I would just comment on the existing threads, but hte reputation gate wont allow me.
I also ran into this problem a few months ago on Ubuntu Gnome where the screen would go black before I could enter my password. The same this is happening here in Unity after running a system update and then rebooting. I installed Unity on my laptop a few weeks ago and used the Nvidia proprietary driver as indicated in the Software and Updates window under Additional Drivers. Using that I have no problems. Only when I add the xorg-edgers ppa to my repositories and then executing a system update as notified does this problem occur. I used the same ppa in Ubuntu Gnome which generated the same problem. This time in Unity I didn't even install anything form the repository so I don't know why this happens. I could be making assumptions based on the correlation I see between adding the ppa, running updates, and then rebooting with a black screen before login.
How do I get access to the terminal to start trouble shooting the black screen nonsense if I can't get a login screen? Lately my beset solution is to clone the drive before this nonsense happens and image it back if it does break.