Sounds ugly!
When this happened to our 'box, we had to remove it, strip it to find that the circlip groove at the back of the main shaft was stuffed ( it is cut into a spline which is not terribly good). This allows the mainshaft gearset to move fore and aft and it jams.
We fixed it by machining the section down then turning up a short push fit sleeve to retain and support the circlip. For absolute safety, we also fitted another circlip at the outer end of the sleeve (cut into the main shaft) to prevent any movement.
So far the fix seems to have worked.
This is the worst case, I think.
It could be also worn selectors, or a broken selector fork.
In any event, the gearbox has probably got to be hooked out and stripped.
Best of luck!