As the pilot approached his destination he was advised that he was number three in the landing sequence. A visual, straight-in approach was made in clear, dark conditions. After receiving a landing clearance a normal flare was made, and it was not until the aircraft settled onto the runway surface that the pilot realised that the gear was retracted. At the end of the landing slide, the pilot turned off the master electrical switch and vacated the aircraft. He had made no attempt to alert the Control Tower to the fact that the aircraft was disabled on the runway. The controller, not seeing any lights indicating that the runway was obstructed, cleared a waiting Cessna 172 for take-off. Only prompt action by the instructor in this aircraft prevented a collision with the stationary aircraft.