[Adoby]

Just restore a backup of your Calibre library.

You DO have a backup, don't you?

Otherwise it should be possible to create a new library with a new name and let Calibre import the files from the folder with the old library. If you get the latest metadata is not certain.

Start by making a copy of the old calibre library, in case something goes wrong. Look inside it to verify that it is full of folders with abbreviated author names with ebook-files inside them.

Check in settings that metadata is read from file contents.

Run "Add books from directories, including subdirectories (One book per directory..." on the new backup copy of the old calibre library.

If all goes well you will have everything back. Then delete the old library and the copy. (But NOT the new library!!!)

Finally, and very important: Make a backup of your calibre library folder. Place the backup on a USB-stick or external HD or to the cloud. And update that backup every time you make changes!

If you value your work on the metadata for the books, and the books themselves, you will most likely have more than one backup...