.
The files are probably still there, but the directory structure is wrong.
Maybe. But from https://www.theregister.co.uk/2018/10/05/windows_10_wipes_files/:
The problem appears to be that data stored in a user profile directory can be deleted. Permanently. Thus users who have cheerfully performed the update found themselves with a lot more disk space but none of their precious files. One Redditor reckoned that all files in their <user>\Documents folder had gone but, on the plus side, there was now an extra 60GB of disk space. Yay Windows.