Sounds very odd.

First thing to do is a battery disconnect: disconnect battery, wait 15min, then reconnect. This resets everything in the car.

Second thing: Do you have a TV tuner fitted? if so, unplug the nav and let the TV take over generating the picture to ensure the rest of the system is OK.

But it may be that the nav unit has failled. You could try sending it to VDO in Wetzlar Germany for repair.