Do you see any scabs on combs, wattles or faces? Yellow plaques inside the beak and throat can indicate wet pox. Pictures can help. Fowl pox is more of a problem during warmer weather when mosquitoes are about. Unless there has been pox in the area or from new chickens where it has been a problem, it can linger in the environment on shed scabs for months. Wet pox is much more lethal, and involves yellow patches inside the throat, airway, and digestive tract.
During winter weather in cold states like yours, I would look at other possible causes. A necropsy performed by your state poultry lab would be a food way to get answers ona cause of death. Sorry for your loss.