I had exactly this problem on possibly the first of our computers to have this upgrade, took me many hours to sort out (finally by downloading a fresh installation of Windows 10 version 1803 from Microsoft to a usb stick).
why did my "invitation" merely talk of new features, and not warn that this was a full version upgrade?
why was I not warned to disable antivirus programmes? (Yes, I use Avast on all our computers)
why is there not a warning to back up all data and programmes before installing these "new features"?
Most unhappy with Microsoft for another forced change which has cost me valuable time, I could have been watching a version of the Great British Bakeoff or planning the route for tomorrow in my HGV , or even sleeping.
I dream of sleeping .....