ABOUT 30 MIN BEFORE THE ACCIDENT, THE PLT RECEIVED AN IN-FLT WX BRIEF FROM EN ROUTE FLIGHT ADVISORY SERVICE (EFAS). THE PLT INDICATED TO THE EFAS THAT HE RECEIVED AN AIRMET REGARDING ICING CONDITIONS ALONG HIS ROUTE. THE PLT WAS THEN BRIEFED ON THUNDERSHOWERS, FREQUENT LIGHTNING CLOUD TO GND, AND PLT REPORTS OF MOD TURB AND LT MIXED ICING. THE PLT CONTINUED ON HIS FLT ROUTE AND 25 MIN AFTER HIS WX BRIEFING REPORTED AIRFRAME ICING CONDITIONS TO ATC. ATC ADVISED THE PLT OF MORE SEVERE WX AHEAD AND RECOMMENDED THE LT REVERSE COURSE. LESS THAN ONE MIN LATER, THE PLT REPORTED BEING STRUCK BY LIGHTNING. ATC AMENDED THE PLT'S CLEARANCE INSTRUCTING HIM TO REVERSE COURSE WITH A LEFT TURN HEADING 330 DEG. THE PLT READ BACK THE CLEARANCE. AFTERWARDS BOTH RADAR AND RADIO CONTACT WAS LOST. RADAR DATA REVEALED THE AIRPLANE ENTERED A LEFT TURN AND CLIMBED 200 FT. THE AIRPLANE THEN DESCENDED ABOUT 1,200 FT IN 35 SECONDS. THE ACCIDENT AIRPLANE WAS NOT EQUIPPED WITH DE-ICING SYSTEMS.