AFTER LNDG AT NEOSHO, MO, THE PLT CALLED THE JOPLIN FSS AT 1132 CST & REQD WX INFO FOR A FLT TO HAYS, KS. THE SPECIALISTADVISED THAT HIS WX COMPUTER WAS OUT OF SVC & SUGGESTED THE PLT CONTACT ANOTHER FACILITY OR CALL BACK IN ABOUT 20 MIN. THE PLT INDICATED HE WAS IN A HURRY & ENDED THE CONVERSATION. NO RECORD WAS FND OF ANY OTHER WX BRIEFING. THE ACFT DEPTDAT 1155 CST. WHILE CRUISING AT 8000', THE PLT ASKED THE CENTER CONTROLLER (CTLR) ABOUT WX ALONG HIS ROUTE. THE CTLR ADVISED HIS RADAR DISPLAYED AN AREA OF LGT SHOWERS AHEAD & CLRD THE PLT TO DEVIATE ARND THE WX. APRX 5 MIN LATER, RADAR & RADIO CONTACT WERE LOST & THE ACFT CRASHED. AN EXAM REVEALED THE L WING HAD SEPD IN FLT FM OVERLOAD FAILURE. THE WING WAS FND APRX 1 MI FM THE MAIN WRECKAGE. NO PRE-ACDNT PART FAILURE OR MALFUNCTION WAS FND. A WX STUDY SHOWED THE ACFT CRASHED IN AN AREA OF FRONTAL ACTIVITY WHERE THUNDERSTORMS (TSTMS) WERE BUILDING. LCL PSNL RPRTD HVY RAIN, LIGHTNING & GND VIS OF 1/8 MI. CONVECTIVE SIGMET 26C (ISSUED AT 1055) WARNED OF IMBEDDED TSTMS IN THE AREA WITH TOPS ABOVE 45,000'.