Hello Intrepid,
this could help,
1. Do you see dead bees that are inside the cell with their rear abdomen sticking out?
This is an indication that the bees starved. If there is honey present in the hive, the bees could have still perished because they were unable to maneuver the cluster over the stored food.
Bees head-first in cells
2. Does the hive seem wet? Are you seeing mold?
You hive could have died from too much moisture. Moisture in the hive can be very detrimental to bees, which is why one should make sure their hive has adequate ventilation.
3. Are there brown stains or streaks on the outside of the hive?
This could indicate a dysentery or nosema issue.
Note.......
For whatever the reason your bees died, it is important to understand why they died. Making sure they have plenty of feed is very critical. Also, having good control of mites is as equally important