What did you do before the message started appearing?
The problem is most likely that your xorg.conf or XFree86.conf (don't know which Debian uses) has been changed. Press ctrl-alt-F2 to get a console window, login as root and type "init 3" to get into non-graphical mode. If your xorg.conf (or whatever it's called) has been changed, try backing up the current version, and then look for another file with the same name but with an extra ~ character on the end. This should be a temporary file that contains the unmodified version of the file. Rename it back to the original name (i.e. remove the ~) (making sure you've backed up the new version first!), and then try restarting the X server by typing "init 5", or rebooting.
If you still get errors, you'll have to edit your xorg.conf back to a working version. Without knowing your setup it's impossible to say what this should be. Search the net - the forums on LinuxQuestions.org
are good and have a search feature, as well as a Debian specific forum.
Hope that helps a bit.