In reverse not moving

Recently took a discovery 1 offroading and while stuck suddenly lost all drive in reverse the gear goes in and the reversing lights come one no loud bang so dont think its blown any ideas

300 or 200 ,lt77 has sliding gear which is slid into gear by a lever which could have come drift or pivot broken ,usualy the gears teeth wear and it throws itself out but that dosent sound like yours box would havs to be split to fix ,r380 has gear in rear extentsion case and is engaged with 5th synchro hub the dog teeth could have become seperatated from gear so that rev gear isnt locked to shaft,can you get all other gears as if you cant other possibilities arise

Its a 200 i can still get all other gears we have attempted to get in through the top however unsure what to do when we reached the top of the box held on my pot rivets. I would rather have to not remove it if possible.

gear box will if actually going into gear but not moving have to come out ,if it is that it is not going enough in to get gear it might be in top housing