Hrtull: consider
freeze out: just as it sounds, the bees froze to death due to not large enough cluster to keep them warm.
Condensation kill: the dead bees will be moist as they died from condensation accumulated that fell on the bees.
Starve out. Bees in the comb head first without presence of honey in immediate vicinity.
CCD: An assortment of conditions that stress bees.
Sorry for your loss. In Arkansas we experience same weather, single digits for a week and then 2 days of 60F, now single digits again for a week,,,,,, and I also lost hives.
Blessings
Time to do some detective work. You have the evidence in front of you, If you open it up and take pictures as go we can assist better. Van has it dialed in but I would like to add the physical indicators of what you may be looking for.
Freeze out will be a small cluster with food available, bees on one or two seams, unable to move to food that is close to them.
Condensation kill, Look for ice or water on the insulation when you open up the hive. Look for lots of mold on the top of the dead cluster. The mold will grow as the water drips onto the live bees and the heat from the cluster makes it warm enough to grow. It will not be extent his time of year for your cold area.
Starve out he addressed well.
Did you do a mite count in the mid summer? Treating with OA now works but only if you can get the mite levels down enough to get them to this point. If they were infested and sick at this point they are done. You get the mites knocked down for winter for healthy bees and in winter when the mites are very vulnerable to soft treatments.
Now if it was mites and you had honey in the other hive your good hive just ran over there, grabbed all the honey and the mites and virus for your surviving hive. OA may save the existing.
You can direct where the water condenses by leaving off a little insulation or an opening to the outside. The warm air will rise with the moisture and leave or hit the cold patch of the wood and turn to water and drip at that point. Keep in mind it may freeze at that point.
Sorry for the long post.