Remove the contents of the freezer as quickly as possible and store it all together in containers (insulated ones if possible). If you have some to hand, pile newspapers, towels or blankets on top to keep out the warmth.
Defrost the freezer as quickly as possible, dry it out and turn it back on. As soon as it feels colder in there that it does in the room replace the contents.
Food doesn’t instantly thaw. Remember, if the power goes off, food in the freezer should stay frozen at least for twenty four hours. Doing what I said the food should stay frozen for several hours, easily long enough to defrost the freezer.