I also have a submersible pump. In the pump house there is a "Pump Tech" controller. So, you should have a pump in a well, with wires going down to fire it up.
Also, there should be a foot valve that keeps the water in the line when the pump is off. In the pump house you either have a breaker box for electricity and or the pump tech that stops the pump when the water gets low. Then many have a pressure tank with an air bladder that provide water pressure to the house and signals the pump to go on and off at a set pressure.
I do not follow your set up with a hydrant and a switch and then to the house. It sounds like your house is running directly off of pump pressure?
I would investigate the pump first and maybe a broken foot valve?
Sometimes a piece of plumbing can get lodged in the pipe and reduce pressure.
The other thing I would check is the pressure tank. Sometimes these lose air pressure or get water logged. that will cause a disruption of water pressure.
Another thing that can happen is "low water" in the well. This will cause a direct loss of water pressure and the pump will cavitate.
That is why I have the "Pump Tech" that turns the pump off with low water or the windings in the pump motor all of a sudden have no resistance.
A water storage tank is a good way to go that fills up with water so that in that you pump water from the tank to the house and not from the well to the house.
This way, the pump acts alone, and fills the tank on its own. Turns off if low water and turns on after water is up. You alsways have a reserve in the tank.
I am not a "master pump person" but explained it in my layman way.
I am sure others will have their input.