Sounds like it's low on fluid. Some where about 1/2 way up on the cylinder should either be a pressed in rubber plug or a screw/small bolt. Remove that and with the cylinder compressed pour jack fluid in there. After you put a little fluid in and try pumping it and see if the cylinder goes up. Keep putting a little fluid in at a time and pumping until the Cylinder is fully extended. Replace the plug and let the cylinder down. Then cycle it a few time.

If it doesn't jack up after you have put fluid in chances are the internal seal(s) are bad.