This article describes that the FortiManager firmware can hang more than 10 minutes after an upgrade.
Basically, when the FortiManager is upgraded, it will take around 5 minutes to reboot the GUI.
Step 1: If the GUI hang, need to check from CLI and ping the IP of FortiManager.
Step 2: If ping success and access via CLI failed, it is necessary to access via console. (The customer needs to have access into console VM).
Step 3: After access into console, the page still could not be accessible. It is necessary to restart or turn off/on the instance off FortiManager.
Step 4: Access again via GUI to check if the firmware been upgrade or not. Ensure, before to perform an upgrade, to take the snapshot or backup config to prevent any issue.