DURING THE RETURN FLT TO STONE MOUNTAIN ARPT, THE PLT CONTACTED APPROACH CONTROL & REQUESTED VECTORS TO THE GWINNETT COUNTY ARPT AT LAWRENCEVILLE, GA. THE PLT REPORTED THAT HE HAD BEEN GOING AROUND THUNDERSTORMS & IT WAS TURNING DARK. SEVERAL MINUTES LATER THE PLT ASKED HOW TO TURN ON THE RWY LIGHTS & STATED HE HAD THE ROTATING BEACON IN SIGHT. A CFI AT THE ARPT OBSERVED THE PLANE FLY OVER THE ARPT AT 300 TO 400 FT GENERALLY ALIGNED WITH RWY 7. THE RWY LIGHTS WERE OUT DUE TO THE MAIN CIRCUIT BREAKER BEING TRIPPED BY LIGHTNING. THE ACFT WAS THEN OBSERVED ENTERING A WINGS LEVEL, STEEP DESCENT WHICH CONTINUED UNTIL THE ACFT WAS OBSCURED FROM VIEW BY RAIN & FOG. THE WRECKAGE WAS LOCATED ABOUT 1/4 MI EAST OF THE DEPARTURE END OF RWY 7. THE FLAP ACTUATOR WAS IN THE MECHANICAL POSITION CORRESPONDING TO A 40 DEG FLAP SETTING.