What was found
IN CRUISE FLIGHT, CAPT'S NAV DISPLAY SHOWED VTK AND LOST ALL NAV DATA. FO'S MCDU DEFAULTED TO MENU FOR 30 SECONDS, THEN ALL RETURNED TO NORMAL OPS. GROUNDED - YES. (RESOLVED ON W/O: 1974809 TASKCARD: N/R-00001 ) REMOVED AND REPLACED CAPT FMC REF MM 34-61-02-400-801, OPS NML REF MM 34-61-00-750-803 AND MM 34-61-00-710-801. THIS CLEARS MEL 34-61-01A, REMOVED PLACARD C/N 26044954, SEAMC " PIDDIE " NOTIFIED. FMC SW MOVED TO NORMAL POSITION. OPENED AND CLOSED TWO C/B'S.