Check KDS' web site for updated monitor drivers / inf files; it is possible the current drivers became corrupt. If possible, check your monitor on a different system, and see if you can duplicate the problem with the 640x480 resolution in safe mode.
If that doesn't help, the monitor should still be under warranty. Contact KDS for an RA number; that may be required at a service centre (there is one in Calgary and Edmonton) or it might speed things up a bit.