Try the easy things first.
Could a remote hyd handle be pulled back (or pushed forward) to the detent? If a remote is engaged you will have no 3 pt lift.
Also, have you tried to bleed it? Loosen (but don't remove) the nut on the pressure side of the pump. Run the engine at idle and let it puke oil till it stops spitting and or bubbling. Then retighten. If it is easier you can also do it from underneath. (second photo)