Isolate her from the others in case it is communicable, bring her into a cooler room or garage, set her up with a comfortable box or cage and provide her with water that has vitamins added (such as chick saver or nutridrench) and a small bowl of scrambled eggs or similar healthy treat.
This will allow her body to spend it's energy fighting off illness without fighting cold weather and it will allow you to observe her behavior, breathing and her poops, this will help you determine what other steps are needed...
Edit to add...don't forget to check her closely for injuries (young hens can be injured by roosters when they struggle) and for mites or lice, which sometimes favor one chicken over others for some reason. While handling her also check her crop size/hardness (to watch for impaction), smell her breath (cor sour crop) and feel her posterior abdominal area for hardness or swelling (internal laying or egg bound symptoms)