I had a situation a few years ago where I killed a few of my queens too late for them to be mated, and while I think one of the worthless virgin queens did survive till spring, the others died off.
I'd guess that is what happened in your case too. They tried raising a queen but the cold got them first, I think they probably have less will to live without her majesty. Plus they couldn't move when the future of the hive was locked up in those 2 queen cells.
Why and how she died? Well that is probably a mystery never to be solved. Probably a heart attack or something.