When a water pump runs continuously, first shut off electrical power to the pump, and then check if the plumbing system still has pressure by opening a nearby water faucet. If the system still has pressure and won't turn off, the pump may not be primed correctly, or the pressure control switch may be improperly set.
If a water pump runs continuously and the system still has some pressure, it is possible that water is running somewhere in the plumbing system. Verify all faucets, toilets, gardening equipment, boilers and anything else that uses water from the main plumbing line. If there is nothing running, a burst pipe may be creating the pressure difference that keeps the pump running. If not, the problem is with the water pump itself, which may have lost its prime.
Verify that the water pump is adequately primed and that the pressure control switch is set to a reasonable pressure. If someone accidentally set it to a higher pressure than the system can achieve, the pump may run continuously.
Another possibility for a water well pump that runs continuously is that the well water level has dropped below the necessary threshold to operate the pump. If the pump cannot deliver water in sufficient volume, the pump's tailpiece may cause water to recirculate through the pump, keeping it running indefinitely.