On July 27, 2022, about 0900 central daylight time, a Mooney M20C airplane, N21007, was substantially damaged when it was involved in an accident near Farmington, Missouri. The flight instructor and the pilot were not injured. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. The flight instructor reported that the purpose of the flight was to conduct a flight review for the pilot. The flight originated from the Washington County Airport (8WC), Potosi, Missouri, about 0820. After a normal flight, as the airplane approached its destination, Farmington Regional Airport (FAM), the flight instructor extended the landing gear. He felt the landing gear drop but he did not see a safe landing gear light indication in the cockpit. He cycled the landing gear 3-4 times and the landing gear down light flickered but did not indicate a safe down condition. The flight instructor elected to land the airplane as gently as possible on the runway. During the landing he felt the tires touch the runway and the landing gear collapsed a few seconds later. The airplane slid on the runway on its belly, which resulted in substantial damage to formers and stringers below the belly skin. A local mechanic lifted the airplane and the gear down lights did not illuminate. He secured the landing gears with bars and towed the airplane off the runway. The landing gear system was examined on December 5, 2022. During the examination the landing gear mechanical linkages were found to be functional, and no failures or abnormalities were found. When the master switch was turned on, there was no landing gear light indications for the up or down lights. It was discovered that the owner partially removed the instrument panel after the accident, and it was no longer grounded. A jumper wire was used to complete the ground from the instrument panel to the airframe. At this point, the gear was fully down and the green gear down indicator lights illuminated. The gear handle was moved towards the up position and all three landing gear went to the up position. The landing gear was successfully moved up and down multiple times and no malfunction was found. The cockpit landing gear up/down lights functioned normally. The throttle position warning horn was checked by pulling the throttle back toward the idle position and verified that the horn was functioning. The entire landing gear system performed correctly, and no faults were discovered. A review of the airframe logbook entries did not reveal any discrepancies or uncorrected defects with the landing gear system.