The airplane was in level cruise flight during dark night conditions when the 'engine stopped.' The instructor elected to make a forced landing to a field. On final approach, the instructor noticed what appeared to be power lines. The instructor banked the airplane to parallel the power lines and rotated the airplane to a nose high pitch attitude for touchdown. The touchdown was made on both main wheels; however, 'the nose of the aircraft came down hard,' and the airplane skidded to a stop. An FAA inspector supervised a 35-minute test run of the engine and found no anomalies.