Sounds like a failed diaphragm in the pump
Assuming of course that it IS a diaphragm pump. If it's original, it's 20 years old, which is about when most of 'em do fail. When the diaphragm in a pump fails, it causes the pump to pull in air, which causes the pressure to drop, which causes the pump to run continuously without pulling any water through it.The first warning sign that the diaphragm is going: the pump cycles for no reason...at first just a short brrrp that can be hours apart, then closer together and a little longer...till eventually it starts to run continuously and shortly afterward ceases to pump any water.Allowing it to continue running WILL burn out the motor. So turn it off and the breaker (or pull the fuse) till you can either replace the diaphragm or replace the pump. If the pump is original, I'd opt for a new pump, 'cuz a complete rebuild kit--diaphragm, pressure switch etc (which it really needs if it is original)--will cost you almost as much.