The pilot reported that he departed from a grass airstrip in "wet conditions." He noted that the short 15 minute flight was uneventful. However, upon landing, both main wheels on the tail-wheel equipped airplane would not turn. Full aft elevator control was not sufficient to maintain control and the airplane nosed over. The airplane sustained substantial damage to the vertical and horizontal stabilizers. The pilot reported that the brakes were "firm" when checked on final approach. He added that immediately after the accident the main wheels could not be rotated. Although, approximately 45 minutes later, they were free to rotate. The pilot noted that the temperature was about 35 degrees Fahrenheit. He recalled thinking that water which accumulated on the brakes during takeoff may have frozen during the flight.