Flight was receiving VFR flight following thru the Los Angeles TCA. At about 1620, flight was handed off to another sector controller and given a freq change. The pilot acknowledged. He did not contact the new sector controller, and no further radio communications were noted. Recorded radar data shows that the aircraft maintained a straight ground track at 5,500 ft during the handoff and freq change assignment. At 1622:37, the altitude changed to 5,300 ft. During the next 18 seconds the altitude decreased to 3,100 ft at an average rate of descent of about 7,000 fpm. The last recorded radar position is over the Pacific Ocean mid channel between the mainland and Catalina Island. There were no known maintenance discrepancies against the aircraft, and the aircraft is estimated to have had about 34 gals of fuel on board. The pilot had been involved in an automobile accident 1 week earlier; he had reportedly fell asleep while driving. He also reportedly had 'very bad bruises on his body in the area of the seat belt and shoulder harness.'