On March 5, 2000, at 1200 eastern standard time, a Cessna CE-560, N314QS, nose gear collapsed during a normal landing at the Ocean Reef Club Airport in Key Largo, Florida, The on demand air taxi positioning flight was operated by Executive Jet Incorporated under the provisions of Title 14 CFR Part 91 with a flight plan filed. Visual weather conditions prevailed at the time of the incident. The airplane sustained minor damage, and the two air transport pilots were not injured. The positioning flight departed Palm Beach, Florida, at approximately 1130. According to the pilot, the before landing check, which included the lowering of the landing gear, was completed several miles on short final. The pilot recalled that the landing gear indicator lights were green or indicated that the landing gear was down and locked. After the airplane landed, the pilot deployed the thrust reverses. At approximately the same time, the nose gear retracted into the wheel well. The airplane slid down runway 04, and came to rest 2000 feet from the end of the runway. The preliminary examination of the airplane disclosed that the nose gear up-down-lock actuator assembly experienced a mechanical failure. During the subsequent examination of the assembly no mechanical problem was detected during functional testing. All functional pressures and other operational parameters tested within the normal certificated limits.