This Ubuntu 11.10 bug and solution is described in:
http://uksysadmin.wordpress.com/2011/10/14/upgrade-to-ubuntu-11-10-problem-waiting-for-network-configuration-then-black-screen-solution/
Shortly the solution:
- Hit Ctrl+Alt+F1 at the blank screen to get you to a non-X terminal (tty1)
- Login in with your username and password
- Change to root with: sudo -i and enter your password
- mkdir -p /run /run/lock
- rm -rf /var/run /var/lock
- ln -s /run /var
- ln -s /run/lock /var
- reboot