Do you have any pictures? Frostbitten combs usually heal without treatment. If you can bring her inside where it is above freezing, you could use some hydrogel spray, under brand names of Vetericyn, Theracyn or other wound sprays. It may take a week or mire to see how much damage there will be. If the points of the comb are frostbitten, there may be some rounding off of the comb. Make sure that your coop has some room over the roost, and some ventilation so that humidity can dissipate.