Before buying solenoid check to see if you have 12 volt to the selenoids or not. If it is a Power gear setup, there might be 3-4 selenoids and valves involved to operate the system. The "dump" valve activates and allows fluid to flow back into the reserve tank and thus raise the jacks. It has to have 12 volt to work.
Does it have springs to raise the jacks? Sounds like it does since someone released the pressure to raise the jacks. In that case, the selenoids need 12 volt power to operate. Somewhere there are fuses to check and verify 12 volt.
In order to really help, we need more information as to the make of the system. ID numbers? Maybe a picture of the system? I just went through the same type of problem and I would advise to check out some other things before throwing more money at it.
There is a lot of help here on the forum if you can provide more detailed information on the type of jacks you have. Power Gear has a great manual you can download if that is your brand. And lastly a solenoid can be purchased on E-Bay or from NW RV Supply in Oregon. Best of luck
Arnold
