Reboot. When GRUB starts, don't wait for it to time-out... Hit the space bar. Now, choose the first entry and hit the E key to edit it. Select and edit the line starting with "kernel". At the very end, and a " 3". Hit enter to finish editing, and B to boot. (I'm writing all this from memory and it's been awhile, so read the instructions as they appear.)
This should boot you into the command-line. From there, you can re-configure your /etc/X11/xorg.conf file. There's prob'ly a xorg.conf~ or something similar right next to it, which would have been your previous conf file.