Comment 48 for bug 811441

got this error after upgrading from 11.04 to 11.10 and couldn't boot up anymore.

did what was in this post and solved the problem:

http://uksysadmin.wordpress.com/2011/10/14/upgrade-to-ubuntu-11-10-problem-waiting-for-network-configuration-then-black-screen-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