Why Did My Bees Die?
Too often the beekeeper will eagerly open up the hive after winter and to their dismay, discover their hive has died. It’s important to understand why your hive died in order to learn how you can effectively keep bees. Many reasons and scenarios are present when trying to describe why your bees died or could potentially die. The most common reasons a hive has died is because of either a mite infestation or starvation. Other reasons include Nosema disease, condensation within the hive and of course plain coldness.
You should conduct an inspection even if the hive is dead in order to determine how the hive died. Here are some clues to keep an eye out for:
Check the bottom or debris board. Do you see a lot of Varroa mites? Your hive could have suffered from a Varroa mite infestation.

Look at the bees themselves. Do they have short abdomens? Do they have deformed wings? This could mean your bees died from deformed-wing virus. This virus is vectored by Varroa Mites.

Do the dead bees look shiny, greasy or darkened in color? This is a characteristic for a paralysis virus like chronic bee paralysis, also vectored by Varroa.

Do the wings on the bees appear to be split or in a K shape. This could be an indication of K Wing Virus which is vectored by the mite.

Are there many dead and dying bees in the front of the hive? Is their tongue sticking out? This is a good indication that poisoning with a pesticide has occurred.

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.

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.
Are there brown stains or streaks on the outside of the hive? This could indicate a dysentery or nosema issue.

For whatever the reason your bees died, it is important to understand why they died. Making sure they have plenty of feed and plenty of bees prior to winter is very critical. Also, having good control of mites is as equally important.