Looks like there is no simple fix.
When the camera went dead on my trip I contacted Fuji immediately and asked for advice as to how to restart. The initial suggestions were to try different batteries and cleaning contacts. This didn't work so the only next step was to send the camera for repair on my return to Canada.
The result is in this email message from Fuji after about 10 days .... "Hi Brian, the camera has been repaired. We replaced the main board and updated the firmware".
I have it back and it is working. Repair done under warranty - no charge.
Should clarify that I was quite familiar with the X camera firmware update process since I had traded in an XT2 for the XT3 and had gone through several upgrades on the former. For that reason, the possibility of an upgrade failure never crossed my mind. There are almost 300 views of my post - no replies until ArtisticiQ with the same problem! With this level of interest I am wondering if anyone else has experienced this problem as well?