Ubuntu was working fine until I went into the Users and Groups dialog and changed my username and password. Now I am getting authentication errors everywhere. For example, if I want to use apt-get sudo, it tells my that my username is not in the sudoers list. Also, I can't get back into the Users and Groups dialog, nor a million other things that require a password. Can this be fixed? Also, how would I be able to log in as root in Ubuntu?

Thanks.

You may be able to boot up into single-user mode and do the repairs from there. Try rebooting and selecting the recovery mode kernel from the GRUB menu. If that doesn't work you may be able to use a livecd like Knoppix to boot up and do the repairs. To enable the root user, you need to edit /etc/passwd. If I remember, the shell and/or password are disabled. If you google, it should come right up, but I don't think you'll need it if you can get in and change your username back somehow. You could also add your new username to /etc/sudoers.

I logged into the recovery mode and as root typed in usermod -l newname oldname and was able to change my username back to what it was before. Everything works fine now, thanks!

Excellent. Enjoy Ubuntu! I love it, personally.