Hi
If you find dead bees with their butt's sticking out of the cells, it usually means they starved. Keep feeding sugar syrup, they take kilos of the stuff if they have no stores. Its probably because the colony was on the brink that you have no brood.
This winter I lost one hive from starvation even though they had fondant on the frames, looks like it was too hard for them to deal with. My second hive was very weak but is recovering now and taking lots of syrup, but no brood. The third hive is better and there is brood.
Cheers
Simon