You have to turn off the supply valve to the system and drain all the water from the tank. Drain all the water you can out of the tank by opening a faucet and letting it run until it stops. Then check the air pressure in the tank. If it is zero, then all the water may not be out of the tank so leave the faucet open while you add air. Pump it up to about 5 pounds less than the pump’s turn on point so that the tank is still pressurized when the pump starts.