On May 29, 2005, approximately 0930 mountain daylight time, a Mooney M20K, N928MB, piloted by a private pilot, was substantially damaged when it was overstressed after departing Centennial Airport (APA), Englewood, Colorado. Instrument meteorological conditions prevailed at the time of the accident. The personal cross-country flight was being conducted under the provisions of Title 14 CFR Part 91. An instrument flight rules flight plan had been filed. The pilot, the sole occupant of the airplane, sustained no injuries. The flight originated at APA approximately 0900 and was en route to Camarillo Airport (CMA), Camarillo, California. According to the pilot, he was in instrument meteorological conditions and climbing to his cruise altitude when the horizontal situation indicator (HSI) failed. He became disoriented, the airplane got into an unusual attitude, and he lost control of the airplane. When the airplane broke out of the clouds, it was descending rapidly, and he pulled back hard on the control yoke to arrest the descent. During the recovery, the airplane's structural limitations were exceeded and the airplane was overstressed. According to a telephone interview with a mechanic who replaced the HSI at Duncan Aviation, a repair shop located at APA, a plastic gear in the HSI was "stripped" and the heading bug caused the compass card to "hang-up." According to an FAA airworthiness safety inspector, the wing was wrinkled and the accompanying rivets were "popped." Damage to the wing was such that the insurance company wrote off the airplane as a total loss. The pilot was unaware of the damage to the airplane and had flown it on numerous occasions after the accident. When the damage was brought to his attention, he reported the accident.