As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE WE LOST VNAV GUIDENCE AN THE #1 FMC AND BOTH FMC'S WENT BLANK. WE HAD A VTK ON DISPLAY #1. AFTER A BREIF PERIOD ALL SYSTEMS WERE RESTORED. GROUNDED - YES. PERFORMED B737-800 FIM 34-61 TASK 836 "FMC DOES NOT OPERATE" NO FAULTS NOTED, NO HISTORY, OK FOR SERVICE.