The water was out all day and night yesterday, but when I got up this morning, it was working again. But it was too late, the well guy was already on his way out. He climbed down the hole and looked around. He said the problem was in the pressure sensor; several days of sub-zero temperatures had frozen the sensor in its open position, so the water pump thought the pressure tank was full and wouldn't pump any more water. It warmed up enough this morning to defrost the sensor and restart the pump.
There isn't any sub-zero weather in the current forecast, so hopefully the sensor will warm up enough so it won't happen again. We need to get some kind of heater device we can lower down there to warm things up in case the weather turns arctic again.