What was found
IN CRUISE THE LNAV/VNAV DISENGAGED AND BOTH CDU'S WENT TO MENU MODE WITH THE FMS MISSING ON THE MENU. CAPTAIN DISPLAY BRIEFLY SHOWED A MAP FLAG WE SELECTED A/P B AND ALL STARTED TO WORK NORMALLY CONSULTED QRH BUT EVERYTHING RECOVERED BEFORE WE WERE ABLE TO RUN THE CHECKLIST. GROUNDED - YES. #1 FMC LOCKED UP & HAD A VTK/PGM FAULT. REF B737-9 IFIM TASK 34-61-00-810-847 REBOOTED #1 FMC COMP & #2 FMC COMP. NO OMF ACTIVE FAULTS NOTED. (LOSS OF FMC #1 CAUSED LOSS OF LNAV/VNAV IN FLT) GND VERIFICATION OK.