Plumbers that do not know how to and are not willing to pull pumps etc. should not be used. A pump guy or well driller is the best choice.
You already did all you can to find the problem, a leak between the submersible pump in the well (that is where your pump is right) and the pressure tank/house.
Shutting off the power to a jet pump, if you have one, can cause a loss of prime and no water until you can prime the pump again (a leaking foot valve etc. prevents priming) but.. if you have a submersible pump, shut off the water to the house past the tank, then the power to the pump and see if the hiss stops.
Then shut off the pump and drain the tank and replace the busted gauge so you know where the pressure is. Then start the pump and note the off switch setting. Then run water and note the setting when the pump comes on and note the reading. Shut off the power to the pump, drain the tank, shutting the water off to the house first if you turned it on, and check the air pressure to be 1-2 psi less than the cut-in switch setting; 30/50 psi water pressure gets 29-28 psi air pressure with no water in the tank.