get a terminal line--however ya can. (even with the boot disk and chroot/mount the disk)
delete the /etc/X11/xorg.conf file
rename the /etc/X11/xorg.conf.backup file to /etc/X11/xorg.conf
that's if ya used the display manager setup
otherwise it may be called xorg.conf.bak or xorg.conf~ or something along those lines--unless you haven't been making a backup.