When I have had that happen i had to reseat the connectors at the computer or the monitor. If for any reason the connection was interrupted, then the monitor will no longer be recognized by Windows. After ensuring that the connectors are well seated, reboot the computer, and restart Hauptwerk.
What type of monitors are you using, and what cable type is used for the video? Also, if they are touchscreen, is the associated cable for the touch capability also properly seated?
In Windows settings, click on system, then display. See if the second monitor is recognized. If not, try clicking on the "detect" button under "multiple displays"
Even without Hauptwerk running, the second monitor should at least display a background screen.
Ray