is it like something is keeping you from physically putting it into gear, or can you get it into gear and then nothing happens. and grinding or other nasty noises associated with it?
My first guess in any situation of a single gear not working, particularly if you cant get it into the gear is synchronizers. I have never tried to change them, and other than knowing what they are couldnt tell you much else.
first things to check would be your clutch fluid reservoir and make sure it isnt low, and then make sure some kind of debris hasnt gotten around the shifter linkage and blocking it. I had a rock end up binding my shifter linkage in a old car and could not get it in reverse no matter what I did.
Apply dirt liberally, rinse and repeat
<<<-click for build
cut a bit
Lifted it more
welded a lot
cut some more
Got a winch
Parked until I can get a new steering setup.
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