You can try entering the emergency recovery mode.
Alternatively, boot from a CD or DVD, and mount the drive (say, to
/media/hdd). Then, enter the old system via a chroot with
sudo chroot /media/hdd
Once you're in - either via emergency recovery mode or CD/chroot -, fix the problem by editing
/etc/sudoers (for example, with the command
It should read like
root ALL=(ALL) ALL
%sudo ALL=(ALL) ALL
Also, make sure that your user is still in the
sudo group, with the command
usermod -G sudo -a travis
(Most likely, you executed
usermod -G without the
-a parameter if that's the case).
You can try out sudo by typing
su travis, and then
sudo -s. If
sudo fails, type
exit (or press Ctrl+D to get back to your root shell. After you've fixed the problem, simply reboot your old system (remove the CD/DVD beforehand) and everything should work again.