Can you get to a command prompt? If so, you can try looking in /etc/X11 for a file called XF86Config*. If you're lucky, the old file will be backed up (look for the file with a .old or .bak or ~ suffix). In that case, rename the current file and copy the backup to the current file name. You can also look in /usr/X11R6/bin for a xfsetup program (I can't remember the name right now, but it's a basic XFree setup program which can be run from the command prompt to configure the X server). Remember to login as root to make these changes global.
If you can't get to a command prompt, check during boot up to see if you're boot manager has options and pick one like a Safe option or administrator. These should load with either sensible X defaults or drop you to a command prompt.