assuming this is a hwh system, with the engine running, (hwh is connected to the engine batteries) check all fuses first. (use a test light) there are fuses down by the pump, as well as in the control box. after that, start with a 12v test light again, check the 12v solenoids at the pump. they are mounted on the motor. there will be three solenoids. one is the master, the second is for the jacks, the third is for the rooms. when you turn the system on, there should be 12v on both sides of the master solenoid. and 12v at one side of the other two. (if you have a key operated pad for the rooms, it must also be turned on) when you hit the extend for the jacks, both sides of the solenoid should be hot. when you hit either room extend, there should be power on both sides of that solenoid. if not, check the small stud on that solenoid. there should be 12v there. if not, go up the line checking connections.
if this isnt hwh, you a service manual anyway.
download a service manual for your system. diagnostics are the answer. you cant do good diagnostics without a manual, unless you have lots of experience.