If it was fitted at the factory, comes under standard warranty. If fitted by the dealer, should come under dealer warranty. If VW approved, should come with whatever warranty they offer but will be limited.
You can always replace the entire thing for £400 or so. As above, a few people have managed to disassmble the RNS. You say the screen is blank but the touch screen works? What do you mean by this? Are the buttons up and responding to the presses? Is it just the map what's blank? Please elaborate.