Well I lost one of my birds due to the cold this year it was 16 degrees out i think and I open the coop and she was froze solid by the time I went to let them out in the morning.
I have to agree, I doubt the cold killed her. If it was indeed the cold that killed her, why not any of the others? She had something else going on that killed her during the night and by the time you found her, her body had adequate time to lose all its heat and be in rigor mortis.