I had the same problem, my detents finally just stuck from rust and crud. Was it in the float position? If so I would bet that is what happened. If you have the single stage thrower you can just switch hose hook ups and limp for a while. Look at jdparts, you can see the blowups there. Mine happened to be:AM104228 CONTROL VALVE 1 (SUB FOR AM100921) (SUB AM108779)
I put the tractor up on ramps under all four wheels. I simply pulled the belly screen, released the pressure on the system by moving the levers back and forth. Next I put a packing blanket on the floor (to catch parts) and crawled under and spun out that cap. When I pulled it out I found a bunch of crud in the end from moisture getting in there. After I found my lost detent balls, thanks to the blanket, I cleaned up all the parts, greased the whole thing, used the grease to hold the balls there till I could get it back in.
Now for the 64 dollar trick. Use a small pipe clamp to hold the balls in place once they are in the detent position as you slide it all back into the valve. The clamp just needs to be tight enough to get it all into place as it goes into the valve body. Thread it back in and try it out.
Once I figured that out, it was simple! You could also pull the valve out completly, but this seemed much easier. Just don't lose any parts! Search out - I think - stuck lever, someone gave me some good hints when I had the problem....