Cherry, I'd be interested in more details about the set up of that colony. When you cleared off the entrance was it a chunk of ice in the entrance or was there a gap? When was entrance blocked, early winter, late winter...? Overwinter here in upstate NY the bees exhale about 5 gallons of water. Without an upper vent or a lot of insulation on top that water condenses on the cover and drips down on the bees, in bad cases killing them. Also, we have capped brood in the boxes now (based on an indoor observation hive; about 1 week earlier than normal). It's a small patch of brood but it means the colonies are maintaining the center of the cluster at 95 degrees F. I don't think the lack of bottom ventilation would cause the bees to die from heat or moisture. If there is really no ventilation they could suffocate. Also dead bees on the bottom board, besides natural attrition, could indicate disease without flying days. (I have not seen a colony suffocate so I'm not sure if they would all break cluster and be scattered throughout the hive and on bottom board.) Also, when the bees die they are still wet inside. When they die of moisture issues (like no upper ventilation) the cluster is wet. When they cold starve (not close enough to stores to be able to reach them) and are left intact until it warms up, the bees in the cluster mold Even If No Condensation Dripped On them. So based on the description you gave the bees died of 1) disease (mites) with no flying days, 2) condensation issues due to inadeqate upper ventilation or insulation, 3) suffocation. If the entrance was blocked late in the winter and there was no sign of brood I think the colony was already dead. Of course, there is certainly more that you observed at the time that is not in your description above that may paint a totally different picture. I am writing this so someone reading it in the future will have a bit more to look for while doing a post mortem.... Happy beekeeping!