Sounds like a cable malfunction. Or possibly there's an obstruction. Maybe a small stick is wedged in there. It happens a lot with any off road vehicle. Usually its just wedged harmlessly though. But sometimes it'll bind up something important.
So you need to get underneath while someone tries to shift. Do NOT force it. Just gently back, and forth while you watch the linkage try to operate. See what moves, and follow the cables, or linkage. Check from the shifter, all the way to the transmission. It sounds like there's a couple of cables involved, since hardly anyone uses mechanical linkage anymore. Make sure you're following the proper cable. Its usually not hard to find the problem. Cables typically fail on the inside though. So keep that in mind. Cables cannot be repaired. They're cheap enough, so they just get replaced.
If its certain that there's no obstruction. And the cables are in good working order. The next step would likely be the transmission itself.