I can hear the pump motor come on, but there is no air or water coming out directly above it. The system has run flawlessly for five years. In the recent weeks, the pump has been chattering a bit as it turns off. The pressure switch seems to have drifted a few degrees out of plumb, tilting towards the "off" side, and that and the vibration from the pump seems to rock it on and off a few times. But it was still working fine. Then the pump just quit producing any pressure at all. There is a hose bib just above the pump, before the pressure tank etc, and if I open it there is just dead air.

Basic info. The pump is a Myers (ST7-12) well pump, suspended in a water tank. The water tank is always full, it is on city water with a float valve. The elevations - the water tank is at 545', the main house 580', and the highest hose bibs are at 650'.