Are you saying you still have the "NAV" menu in the active info display but when you select it, it doesn't actually show you any maps ?
If so, assuming you have the standard 8" Discover media head unit, then it can't display maps on the head unit and the active info display at the same time (unlike the Navigation Pro 9.2" which can). If this is your problem, simply go to the Nav screen on the head unit, and in the row of icons at the bottom of the screen, I think it's third from right, there is a toggle button that flips the map display from the head unit to active info display and back again. Try that.
On the other hand if the Nav menu has completely disappeared from the instrument cluster, then I don't know. Think this might be a dealer fix ?