Not encountered this one yet.
The shelf works in exactly the same way as a roller blind so it's probably that the spring inside has come adrift.
It's possible that you can get inside the ends to see what's gong on and effect a repair too.
Let us know how you get on.
I'd take a look at mine, but Tank is in the operating theatre at the moment, having his doors replaced. It's been 3 1/2 days without an A2 and it seems a lifetime, especially as I've got a VW Polo 1.2 SE as a courtesy vehicle.
What a cr*ppy car it is - my other halfs 7 year old Yaris is better equipped and rides better too!
Cheers,
Mike