For the second time, I’ve had to reboot my laptop (Win10).
By doing that, I’ve (again) lost the hours spent setting up my library (covers, titles, etc.).
How can I save somewhere all this work for avoiding new hours doing it again? I’ve done all steps of a “how to do it” found on the web, but clearly it didn’t work.
Still not sure exactly what you did. If you reinstalled Windows and wiped out the old documents and application data then your Plex data is probably gone. Plex does not save posters and other metadata in the location of your media files, it uses its own storage space, like other Windows apps.
If your old Windows install is still on a disk, then your Plex data can probably be restored.