WHILE EN ROUTE ON AN IFR FLT PLAN AT FL180, THE PLT ACKNOWLEDGED INSTRUCTIONS TO DSCND TO 16,000'. LATER, THE RADAR CONTROLLER (CLTR) NOTICED THE ACFT HAD DEVIATE FM COURSE & WAS NOT REPLYING WITH MODE 'C' ALT INFO. THE CTLR REQD ALT INFO & THE PLT RESPONDED, '. . . I'VE LOST AN (AWFUL) LOT OF ALT, I GOT MIXED UP IN HERE & LOST ALT, I'M GOIN' BACK UP . . .' WHEN QUERIED, THE PLT RPRTD HE WAS AT 9000' & ACKNOWLEDGED INSTRUCTIONS TO CLIMB TO 14,000'. SHORTLY THERE- AFTER, THE ACFT ENTERED A SPIRAL, THEN RADIO & RADAR CONTACT WERE LOST. AN IN-FLT BREAK-UP OCCURRED & THE WRECKAGE FELL OVR A WIDESPREAD AREA. AN EXAM REVEALED EVIDENCE THE WINGS & RGT HORIZONTAL STABILIZER HAD FAILED FM OVERLOAD, WITH THE WINGS SEPARATING IN AN UPWARD DRCTN. A WX STUDY INDCD THERE WERE CLD LAYERS FM ABOUT 14,000' TO 20,000' WITH LGT TO MOD MIXED ICING IN CLDS ABV 14,000 FT. ALSO, LGT TO MOD TURBC WAS FORECAST BLO 20,000' & WIDELY SCATTERED, IMBEDDED TSTMS WERE FORECAST IN AN AREA NORTH OF THE ACDNT AREA. WITNESSES RPRTD CLDS & OCNL LIGHTNING TO THE EAST.