Now that is strange....
Have you been trying to update it?
Try driving it for a few days, they have been known to crash then return on their own. Failing that the nav may have failed and the TV module will take over the screen so you cant see anything nav related.
If its the latter then you may be able to recover it by opening up the dash, disconnecting the module for at least 15min before reconnecting.
Not sure if disconnecting the battery would do the same thing, someone will know.....