I believe that my pumptro D switch is bad, but I'm not absolutely sure. How can I tell if the switch is bad? It's over 20yrs old, and the pump in the well is running for sure (I can hear it and feel it vibrating), but the bladder pressure is on zero. Usually I can hear the switch engage, but today it stopped and after repeatedly turning the breaker on and off it still won't work.

How much does a new one cost?

You say the pump is running but you have no pressure. Then you say the switch "stopped" and resetting the breaker did no good. If the pump is running, then the switch is working, at least somewhat. The switch controls the pump cutting off/on, so the first thing to do is be sure the points on the switch are closed. Remove the grey cover from the switch and look at the points. Are they closed? Get back with an answer and we go from there.