While upgrading from Ubuntu 13 to 15, about 75% of the way done, the screen went black and wouldn't recover when I moved the mouse. After an hour of the computer sitting idle, and still obviously powered on, I restarted the machine manually. From here Ubuntu doesn't load fully, and I'm sent to a black screen that asks for my login and password. Unfortunately it doesn't respond to the login I'd previously been using. When I boot the computer I've tried restoring a previous version, however this returns me to a similar screen.
What does one do in a situation like this?
