I am a first year newbie with what appears to be a dead hive. I started mid summer with a 10 frame med nuc and got them built up to another deep before fall. I had to feed sugar syrup heavy into fall to get the last of the comb drawn out. I had not been into the hive since Oct until yesterday. I popped the top cover expecting to see the cluster, but they were all in the bottom box. The temp was only 25, which I realize is pretty cold, but I wanted to quickly throw some fondant on in case they were out of food. What I found was the top box apparently untouched, still capped in all frames. The cluster is in the lower box still on the frames and the lower entrance is clogged with frozen bees. I heard no noise and saw no movement. I am assuming this is a dead hive. What should I bee looking for to learn from this experience and to possibly figure out what went wrong?