What was found
THE AIRCRAFT WAS GROUNDED. AUTO PILOT-LNDG ALT MESSAGE APPEARED. ACCOMPLISHED CHECKLISTS WAS UNABLE TO RECOVER VNAV WINDOW. ALL DATA DROPPED FROM FMCS. NO PINK LINE INITIALLY WAS HARD RESET. THE FLIGHT DECLARED AN EMERGENCY AND RETURNED TO LAX WITHOUT FURTHER INCIDENT. REMOVED AND REPLACED LMAT PER W/C 45-92-03. UPLOADED SOFTWARE TO LT AIMS MOO2 PER 31-41-05. STATUS MSG SELF ERASED NO FAULT NOTED.