Wow, this is great feedback from all of you. I greatly appreciate all the comments.
I was finally able to upload the pictures, they may help. Also, I tried to provide additional clarity to the different questions you asked.
Starting with pesticides. It is a possibility, however, my neighborhood if fairly green and we are surrounded by organic farms (5 to 10 miles away). Also the bees seemed health through November.
The queen was not among the dead bees
I did feed them protein patties in the fall and good pollen was available. During that period the hive did seem healthy to me.
This hive did not die of EFB (see pictures), as of ABF, definitely not the cause (see pictures).
As for Varroa mites, checking on the dead bee (see picture) I did not notice and deformities, however, not sure what to look for. Also I did not treat for mites this fall.
Regarding Nosema I did not perform a nosema squash (was is that?), also there were no brown spotting on the outside the hive (I was told that this was a good indicator of nosema infestation (is this correct?).
I had allowed for ventilation in the hive and did not notice any effects of humidity (no wet bees, no soggy inner cover, I have wooden telescopic covers).
Too much ventilation may have been an issue (although I have a solid bottom), we did have some pretty cold days.
Finally there were no brood in the frames.
I am trying to understand if there was anything I could have done to prevent this.
Thank you for your help and insights.