Could have been something else. For instance if they didn't have enough room and swarmed in the fall, that would have been bad news for that hive. Or mites, or if you live somewhere that gets early fall hurricane flooding action, like we did last year, the govt sometimes does mass spraying for mosquitoes and if you haven't registered you hive location with the local State Dept of Ag's no spraying program, then they might have been poisoned. Even if you don't get hurricanes/flooding, register anyway if you live close (like within 2 miles) to agricultural zones.

And you can't just assume there is enough honey for them to make it through the winter. You have to check on them periodically, but don't open the hive, even for a minute, unless it is at least in the high 50's temp-wise. The other way to check is weight. If you try to lift up back of the hive off the hive stand and it is not heavy, you need to feed those bees ASAP.

Also, now that you have an empty hive with lots of drawn comb, first check to see if the wax moths have ruined it all yet, and if not, bait that thing and see if you can attract a swarm to come take up residence! A tiny little bit of lemongrass oil on a cotton ball, placed in the back of the hive will draw them, if there are any scouts in the area looking for a new home. Refresh it every week.