By far the simplest solution is to roll back to the previous Windows 10 build WITHOUT messing about with or moving any of the files. The Update section of settings gives you a helpful link to do this and steps you through the process. The cause is an incomplete install of 1903 (which can be caused by legacy Zune files etc as described by others) and leaves you in a temporary .000 user directory. Rolling back gets rid of the temp .000 user and restores most things.
The only corrections I had to make was in cloud-based apps that could no longer locate directories, including BT Cloud, Google Drive /Backup and One Drive. Once I had restored the previous Windows 10 I had to manually redirect these apps using a right-click on their taskbar icons to the correct folders in my User section, as the failed install had redirected them to new folders within the temporary .000 user.