Determining cause of death of my hive

Hi all,

I was very sorry to discover my first hive, carnolean/Italian bees, who had a very prosperous season, had died this winter. Upon further inspection, though, I have no idea why they died. They seemed to be frozen in place, lots still in there and completely still, others fallen. All dead.

Thought: They starved. True, they did not have extra bee feed (ie bee bread) but when looking they had at least 2 frames nearly full of honey.

Thought: they got a disease. Possible? Not sure how to determine.

Thought: they had mites. Unlikely, as I treated them for mites this October. Also, was not able to see any mites.

Thought: they froze to death. Likely, as it got very cold (down to -10f / -23c)

Any suggestions on how to determine the cause? And how to help them survive in the future?