With the pump hooked to power trip the microswitch on the front of it manually, this is assuming allot first, like its a diaphragm pump in the shur-flo design.
On the front micro switch there is an indentation facing the diaphragm, put a small screw driver on the micro switch cam and press down, then listen for a clic, no clic? (some models are sealed and this cannot be done) or/next test the switch and see if its closed at all electrically with a light
tester or vohm. The pump starts in the closed position, no circuit there? then replace the switch.
You may just want to jumper the two wires at the front of the pump, its easier to also test there, if it starts its the switch.
Always establish you have ground to the pump and + is live when you test, saves you owning switches when you do not need them.
This switch is a known issue and often sticks if unused any period of time.
Happy Camping Safe Trails.