The pilot reported that he was unable to start the engine using the cockpit's engine controls. He suspected that the starter drive gear was not engaging the propeller ring gear. He exited the airplane to rotate the propeller by hand in order to manually engage the starter's drive gear to the propeller ring gear. As he rotated the propeller, the engine started and the unoccupied airplane proceeded across the airport, colliding with a hangar. The pilot reported that after the accident he found the airplane's ignition key was selected to one magneto instead of being in the full-off position. He also remarked that he had left the mixture and throttle levers in the full-forward position and that he had not used the parking brake. The airplane's wings, vertical stabilizer, rudder, and fuselage were substantially damaged.