I upgraded my Ubuntu desktop yesterday from 10.04 to 11.10 and ran into some pretty significant problems. If you get an error saying 'Waiting for Network Configuration', and it sits there until it finally goes on to a black page, then try the following:
CTRL+ALT+F1 should get you to the command line. Do not try to get into the command line from the recovery option, or you will run into 'read only problems'. From the command line, log in as root or a sudoer (your main user account should be in the group) and do the following things:
1. create directories /run and /run/lock,
2. move contents of /var/run into /run and /var/lock into /run/lock,
3. delete directories /var/run and /var/lock
4. create replacement simlinks; e.g. 'ln -s /run /var/run' and 'ln -s /run/lock /var/lock'
For less experienced users, you will need to type the following commands:
sudo mkdir /run sudo mkdir /run/lock sudo mv /var/run/* /run sudo mv /var/lock/* /run/lock sudo rm -rf /var/run sudo rm -rf /var/lock sudo rm -rf /run/dbus/* ln -s /run /var/run ln -s /run/lock /var/lock
After that reboot your computer:
Ubuntu *should* now start up properly.