If you can start the computer then go to a terminal programme and copy the
text within single quotes (without the quotes) at the command prompt. Press
enter, type your normal user password, press enter and it should get you
back to square one. Next time you attempt the upgrade it may work better...
If however you can't start your computer at all you may need to boot from
the original Ubuntu CD, which will allow you to start some repairs, but I'm
not sure if that command string will work then - probably not. Might be
easier to just re-install, unless you have done a lot of customization.

You didn't state which version you are using. I'm no expert but I think I
heard that you can only go up one version at a time, i.e. from 8.10 to 9.04.
Ubuntu upgrades are issued twice a year in April and October. Current
version is 9.04, "Jaunty Jackalope" (possibly misspelled!), but 9.10,
"Karmic Kangaroo" will be available around the end of this month. Each one
brings some new features and inevitably a few bugs that need to get worked
out.