Did my chicken freeze?

Last week the temps went down to 15 degrees, and in the morning, my RIR was dead on the floor of the run underneath the coop. There were no signs of stress or injury of any kind.
I always check to see if they are all in the roost, but it is possible I counted them wrong and she was stuck underneath when I closed the hatch. But, since it was 7 pm when I closed it, would it make any sense that she would not have gotten up to the roost on her own?
There is lots of straw in that area of the run, and it is reasonably protected as it is under the coop. She had not tried to bury herself for warmth; she was simply plopped on her side. Did she freeze for sure? Is there any other explanation?
My sister who is a vet tech took a look at the body and noted that it was not frozen all the way through and no signs of any other problems.

...also...the next day the other chickens were somehow slower, not eating as much and not running around as much. They are back to normal now. Theories??