Whenever you are working at this level things can go awry. However, your
installation is intact. You just need to get X up and running again. If worse
comes to worse you can always boot the Live CD and edit xorg.conf manually.

It can be found at /etc/x11/xorg.conf. You will need to use sudo gedit before
the path to edit the file. However you may want to read the man pages first.
Type man xorg.conf in a terminal or in Firefox or go to:
www.x.org/archive/X11R6.8.0/doc/xorg.conf.5.html

Generally you need to run: sudo dpkg-reconfigure xserver-xorg and you did this
and it messed you up, but that should not deter you from trying again. It is
just a matter of answering questions and if you gave a wrong answer once, don't
give the same one a second time. Another option is to boot the Live CD and
configure it the way you want by pressing F4 at boot up. Then once it boots save
the xorg.conf file to a usb stick or something similar. Then copy that file to
your etc/x11 folder. If this does not work then copy the contents of the file
and paste it into the xorg.conf file on your Ubuntu installation. It is worth a
try, since you are messed up anyway.