On August 22, 1993, about 1110 central daylight time, a Grob G103 sailplane, N422BG, sustained substantial damage during a hard landing at Hinckley Airport, Hinckley, Illinois. The solo student pilot incurred minor injury. Visual meteorological conditions existed for the local, instructional flight. The flight originated about 1059 without flight plan and operated under 14 CFR 91. The pilot stated that while on final approach for landing, at an estimated altitude of 20', he heard a loud crack and was struck in the face by something that knocked his glasses off. He then noticed that the canopy was open and broken, and that the sailplane was in a nose low attitude. He said he was pulling the nose up as the sailplane landed hard. A witness on the ground approximately 300 feet away, saw an object coming off the nose of the sailplane as the Grob was on final approach to land. A postaccident examination of the canopy and sailplane failed to discover why the canopy opened/broke in-flight.