Unfortunately, once the damage is done there isn’t much you can do. However, it is possible that although the tops are badly damaged, the roots may be fine since they were insulated by the soil. All you can do is trim away any “mushy” sections of the top, then wait and see if the plants resprout from the roots or remaining bits. This can take some time, so be very patient before you toss them out. Keep them lightly watered and try to protect them from colder nights if possible—a temperature of at least 45F would be preferable—but spun row covers and frost blankets might be a big help.