I had the same issue on my s373d, my first step was to pull the cap off the end of the hydraulic ram which showed an empty tube, after looking at it puzzled for a few minutes decided that the piston had to be somewhere. So I pulled the lift arm assembly off of the frame to allow access to the rear differential area. I sifted through the oil and found the piston and the actuator rod laying in the bottom of the case. To reattach the actuator rod to the lift arm assembly you do need to remove the lift arms from the housing because there is not enough room to reinstall a cotter pin into that assembly when assembled. Put things back together in the reverse order and you should be set, assuming there was no collateral damage from the piston falling into the differential area i.e. hitting the teeth of the differential and damaging the piston seals.
