Spray lock deicer (or in a pinch, windshield deicer) into the key slot. Slide the key into the lock and twist it slightly from side to side until the lock opens. You could also play a hair dryer across the face of the lock. Spray WD40 into the thawed lock to displace remaining moisture and keep it from refreezing. Spray dry lubricant or powdered graphite into the lock to lubricate it.