My neighbor has a hive that was made from a swarm that he captured last summer. When we checked on them yesterday, more than half of the bees were dead. There was plenty of honey stores throughout the hive. There were no eggs, nor could we find the queen. There was one open queen cell found in the bottom box. The cell was located a little more than halfway down the frame from the top (supercedure cell?). No evidence of AFB. No wax moths, no beetles, very little varroa. What could have happened?