What was found
AIRCRAFT WAS GROUNDED: BOTH FLIGHT MANAGEMENT COMPUTERS FAILED IN FLIGHT AT 390. COMPLIED WITH QRH PROCEDURES, FLIGHT MANAGEMENT COMPUTERS CAME BACK AFTER ABOUT 30 TO 40 MINUTES. VERTICAL TRACK MESSAGE ON BOTH COMPUTERS. ADDITIONALLY, FLIGHT PLAN AND ALL DATA DISAPPEARED. LATER RECEIVED MESSAGE ABOUT LOAD CLOSE OUT BEING DELETED AND NOT CREATED AS IF WE WERE ON THE GROUND. EMERGENCY DECLARED, FLIGHT RETURNED TO MIA AND LANDED WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED LEFT (SEE AALA201902019002) AND RIGHT FLIGHT MANAGEMENT COMPUTERS PER MM 34-61-02 AND PERFORMED SATISFACTORY OPERATIONAL CHECK.