Without a model number for your hydraulic jack, I am not able to see the type that you have. A number of different problems could prevent it from working depending on the type that you have. In some situations, the parts needed for repair are as expensive as replacing the jack. It is normally not a good idea to attempt a hydraulic jack repair yourself given the consequences if it fails and the object that it is lifting falls. If you are able to find the model number on the jack, you can check for available parts on the
website. If parts are available and reasonably priced, I recommend that you consider having a technician repair the jack.