[C320-list] water pump won't shut up

[Nat Antler]

My automatic water pump wouldn't shut off when the sink faucet was closed. It happened when the water stopped flowing so I assume the tank was empty. No matter what I did with the faucet valve the pump kept going and the toggle switch on the panel had no affect so I had to open the panel and pull the power wire from the back of the switch. Questions: is the switch bad? does this happened when the tank runs dry? Is the pump bad? Is the control on the pump that activates it when the water is turned on bad? 

Would be most grateful for insights before I start replacing things that may or may not fix the problem. Thanks, Nat Antler 1994-5 320 #161