On June 29, 1996, at 2000 mountain daylight time, a Cessna 172H, N1686F, made a forced landing in a field approximately 9 miles east of Pierce, Colorado, while on an instructional flight. The certified flight instructor, student pilot, and passenger were not injured and the aircraft sustained substantial damage. The flight was being conducted under Title 14 CFR Part 91 and no flight plan was filed. Visual meteorological conditions prevailed for this local area instructional flight which departed Greeley-Weld County, Colorado, Airport at 1900. According to the flight instructor, he gave the student a simulated engine failure by retarding the throttle. After an idle descent of about 1 and 1/2 minutes, the exercise was terminated and power was added. The engine failed to respond and an emergency landing was made in a field. During landing roll, the nose gear collapsed and damage was sustained to one wing and the fire wall. In an interview with the flight instructor, he stated that carburetor heat had been applied when the engine was retarded at the beginning of the exercise and had been removed when the throttle was advanced. He also stated the engine was cleared several times during the descent. An examination of the engine was conducted by the IIC assisted by an FAA airworthiness inspector, and representatives of the airframe and engine manufacturer. No evidence was found to explain the loss of power. Due to damage to supporting structure, the engine was not run during the examination.