Start by checking hydraulic fluid level.
To do that, start engine, raise hydraulic lift to top position to drive fluid into cylinders from pump reservoir. Next set throttle to 1/3 of full speed. Transmission control is in "Park" position. Now remove hydraulic fill cap, which is centered at rear of tractor. These machines use SAE 30 engine oil in their hydraulic systems. If the level is not up to top of the fill tube, add more until it is full. Then replace cap and see if it will move.
Let me know how this turns out; I have an S 16 that is a basketcase and is missing parts; but I found a complete one that IF I get, the current one will be parted. I KNOW the hydro setup on it is good though.