The pilot reported that he was flying about 700 feet above the ground in cruise flight when he felt the control stick "pop" and the nose of the airplane pitch down. The pilot pulled back the throttle and turned off the magnetos. The passenger adjusted the elevator trim; however, the pitch attitude did not change. The pilot then entered a side slip which brought the nose up to level. He straightened the nose of the airplane just prior to it impacting the terrain. The airplane subsequently flipped inverted resulting in substantial damage to the wings and fuselage. A postaccident examination of the airplane revealed the UP elevator turnbuckle became detached at the elevator torque tube. The turnbuckle was not safety wired. New cables were installed in the airplane in 2007 and the turnbuckle most likely loosened over time.