So you used the update manager to upgrade to the new Ubuntu 11.10 that came out a couple days ago and when you’re finished with the process, next time it boots you get a blank screen.
1. Switch to a terminal.
Press “Ctrl+Alt+F1″ if you’re on Windows.
Press “Cmd + F1″ or “Ctrl + Alt_option + F1″ if you on MacOSX.
You will now be on a text based terminal (TTY1), and you will see that your VM has booted fine, you just didn’t have a graphical environment, therefore the blank screen.
2. Login in with your account on this text based terminal by entering your credentials.
3. Install virtualbox-ose-guest-utils.
(Yes, I also had my virtual guest additions installed before so that I could have full screen resolution, but the problem seems to be related to this and now ubuntu has a package on their repos that you need.)
sudo apt-get install virtualbox-ose-guest-utils [enter]