The CFI and student were practicing a simulated power-off forced landing. The CFI reported that at about 300 feet AGL, when the exercise was ended and the student applied power to go around, the engine quit after the carburetor heat control was moved to the off position. Both pilots reported that attempts to restart the engine, including turning the carburetor heat control back to the on position and moving the fuel selector to another tank, were unsuccessful. During the emergency landing in a field, the aircraft encountered soft and uneven terrain. The left main gear separated from the aircraft and the nose gear collapsed. The aircraft was examined by FAA airworthiness inspectors some time after the accident. One of the inspectors examined the wreckage after it was removed from the accident site. The results of the inspection revealed continuity of the fuel system to the fuel strainer, which was loose and did not contain a safety wire. Fuel was found in the carburetor. The fuel selector was positioned to the left fuel tank.