Windows booted up in its original state. How to understand the reason?

Question

Windows

Started the car as always. Win10 booted up as if freshly installed - personalization changed, shortcuts disappeared, programs did not autoload, even a search box appeared on the taskbar. In general, everything is the same as immediately after installing the system, with the only difference being that the installed programs themselves have not disappeared anywhere. But their settings are gone - users, bookmarks, Chrome tabs.
Because there was no time to understand, but it was necessary to work to work, for a start I tried to reboot. And it helped. The second boot happened already as usual and all the settings, along with shortcuts and autoload, returned to their place. The system worked as if nothing had happened.
What could it be and why?

Answer 1

This could happen if the system for some reason could not find/create a user profile. Then Windows creates a temporary profile, which will be deleted after logging out.
If, after a reboot, the profile is in place, then Windows, of course, will load it.
To find out from which directory the profile is loaded, enter the command in the Windows command line:
set USERPROFILE
In normal mode, you will see a path like this: C:\Users\<username>
When working in a temporary profile, the path will look something like this: <username>

Answer 2

The user of the system has changed. Maybe they logged in under the Administrator or Guest account.