Originally Posted by @calibre manual
In order to understand why that happened, you have to understand what a calibre library is. At the most basic level, a calibre library is just a folder. Whenever you add a book to calibre, that book’s files are copied into this folder (arranged into sub folders by author and title). Inside the calibre library folder, at the top level, you will see a file called metadata.db. This file is where calibre stores the metadata like title/author/rating/tags etc. for every book in your calibre library. The list of books that calibre displays is created by reading the contents of this metadata.db file.
There can be two reasons why calibre is showing a empty list of books:
- Your calibre library folder changed its location. This can happen if it was on an external disk and the drive letter for that disk changed. Or if you accidentally moved the folder. In this case, calibre cannot find its library and so starts up with an empty library instead. To remedy this, do a right-click on the calibre icon in the calibre toolbar and select Switch/create library. Click the little blue icon to select the new location of your calibre library and click OK.
- Your metadata.db file was deleted/corrupted. In this case, you can ask calibre to rebuild the metadata.db from its backups. Right click the calibre icon in the calibre toolbar and select Library maintenance->Restore database. calibre will automatically rebuild metadata.db.
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