My mfd2 can be temperamental sometimes and shows my position but no roads. I find it happens when I start the car but don't let the unit fully start up properly before driving away. Being an older unit (with a slowish start up time) I think it needs a good while to get going and identify satellites etc.
Your issue sounds very strange however. Have you tried removing the nav disc, letting it start up without and then restarting it with the nav disc again? I know it seems mundane but might be worth a try
