Try this.
Clean the exposed part of the ram really well so as not to have anything to damage the seal when it moves in.
Tie the control valve in the down position. Use a long bar to pry the linkage to push the ram in. If it doesn't move remove the pin through the ram and pry directly on the ram. if it moves in, the linkage is stopping things.
If it doesn't move, make sure the cylinder is full of oil. If it is empty the cylinder may be rusty. If it is pretty full, Move the valve to the up position. Hook a come-along to the ram and pull it out. If it doesn't move easily it's frozen. Pull it out till it stops. Then measure the length of the exposed ram. Measure the length of the cylinder Subtract and allow a little for the thickness of the piston. That will tell you how far in it can go.