Had this happen at Spa in 2002, couldn't select any gears but could get drive in neutral which very disconcerting. The problem was the gears were picking up on the shafts, all fixed by Road & Race transmissions. Whilst the engine was out I changed the clutch, CRB & pressure plate as a matter of course, however there didn't appear anything wrong with the clutch.
Whatever the problem the engine has to come out.