Chicken Died overnight, unexpected

One of my poor Columbian Blacktails is dead. I went to let them out this morning and thought she was being lazy so tried to shoo her out and she seemed to move off, so i shut the roof and went out for an hour or so. I have come back to find her curled up asleep (or so i thought) but she's dead! Very sad, but concerning as to what has caused this. The temp dropped overnight and we had some snow, but she was a very helathy looking chickenso I wouldn't have thought that would have caused such a sudden death. any thoughts would be great, bit worried about the othe five now (3 Columbian Blacktails and 2 Red Orpington Bantams)

Thanks ...