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.