I'm sorry to say it's probably be internal to the box.
Perhaps the dog clutch, or the splines the dog clutch slides along to engage the gears have been damaged.
On the other hand it may have been forced out of gear by the dog clutch being thrown back, and could have bent something in the shift change mechanism.
Either way it needs stripping to have a look starting with lifting the shift mechanism out.
What engine is it?
Has the Ring been out of the box recently? As that's the major headache in the strip when the thread is corroded.
Had exactly the same problem,
It turned out to be that the gear change rod had come off the selector in the box.
It would select forward purely by pushing down on the selector fork but it wouldn't engage reverse because it couldn't pull it up, if that makes sense.
I ended up having to buy a new bearing carrier and ring as a result of trying to get it all apart!
