While en route during a cross-country flight of greater than four hours duration, the airplane's engine suddenly stopped. The flight instructor, who was assisted by his student, was not able to get the engine restarted. They therefore executed a night power-off forced landing in what turned out to be a cherry tree orchard. During the investigation, six gallons of fuel was recovered from the right tank, and seven gallons was recovered from the left tank. The engine was test run successfully while being fed through both left and right fuel systems. The entire fuel storage/feed system, including the fuel control unit screen and the fuel manifold screen, was inspected. There were no signs of blockage, anomaly, or contamination that would have kept the engine from producing full power. At the completion of the investigation, no clear reason for the loss of engine power had been determined.