ABOUT 6 MIN AFTER TAKEOFF, AT 2138:31 CDT, THE PLT RPRTD LEVEL AT 9000 FT MSL. AT 2140:17, HE TRANSMITTED THAT HE HAD TROUBLE & SAID HE COULD NOT CONTROL OR DISCONNECT THE AUTOPILOT (A/P) WHICH WAS CAUSING THE ACFT TO DESCEND. AT 2141:14,HE STATED HE WAS DESCENDING AT 6000 FT/MIN. ANOTHER COMPANY PLT IN ANOTHER ACFT ASKED IF HE COULD FIND THE BREAKER, MEANING THE A/P CIRCUIT BREAKER. AT 2141:27, THE PLT OF N8CC REPLIED, 'CALL YOU BACK.' SECONDS LATER, RADIO & RADAR CONTACT WITH N8CC WERE LOST. A SEARCH WAS INITIATED, BUT THE ACFT WAS NOT FND UNTIL THE FOLLOWING MORNING. AN EXAM REVEALED THE ACFT IMPACTED IN A STEEP, NOSE DOWN, INVERTED ATTITUDE WHILE AT HIGH SPEED; MUCH OF THE WRECKAGE WAS BURIED. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND. WITH THE A/P ENGAGED, BACK PRESSURE ON THE CONTROL COLUMN WOULD HAVE CAUSED THE A/P TO TRIM NOSE DOWN. SUBSEQUENTLY, THE MANUFACTURER ISSUED A PRECAUTIONARY ADVISORY TO ALL MU-2 OWNER/OPERATORS REGARDING PROPER OPERATION OF THE A/P AND THE VARIOUS WAYS IN WHICH IT CAN BE DISENGAGED.