On September 30, 2004, approximately 1445 mountain daylight time, a Cessna 172S, N802SP, piloted by a commercial pilot, was substantially damaged during a hard landing at Jeffco Airport, Broomfield, Colorado. Visual meteorological conditions prevailed at the time of the accident. The local instructional flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot and flight instructor were not injured. The flight originated at Broomfield approximately 1400. According to the accident report filed by the pilot, he was a CFI candidate and his flight instructor was preparing him to take the flight instructor practical test. He was asked to demonstrate a short field landing on runway 11R. The airplane crossed the threshold at 65 KIAS (knots indicated airspeed) and was flared about 60-61 KIAS. The nose "suddenly pitched down" and the airplane struck the runway "hard." The pilot added full power, retracted the flaps to 20 degrees, and rejected the landing. The airplane vibrated as it climbed 300-400 feet per minute to pattern altitude. The flight instructor assumed control of the airplane, declared an emergency, and told control tower personnel that the elevator was jammed. The instructor used power to control altitude and flew a 2-mile final approach. The engine was shut down just before touchdown, and a successful landing was made on runway 11L. Postaccident examination disclosed the propeller had struck the runway, and the nose wheel was crooked. The nose strut was pushed back into the firewall. The elevator and elevator trim, which had been set for landing, were also jammed. According to a repair company that examined the airplane, there was no evidence of a pre-existing failure of the elevator. Movement of the elevator and trim cables were restricted as a result of the hard runway contact.