If they get too cold sometimes they will try to hibernate(estivate). You don't want them to do this until they are over a year old as recomended by breeders. You must maintain correct temps and humidity levels at all times. If it drops to far they will either try to hibernate or the frog will not be able to digest it food properly become ill and possibly die. You need to unbury him and check to make sure he is ok and then try and keep the climate correct within the enclosure. Temps should be 80 to 85 during the day and 75 to 79 at night. Humidity should remain from 70% to 80%.

Check on him and keep us posted.