My Library is empty, but my books are still on my computer. What happened?

Usually the books folder moved: OneDrive on Windows may move your Documents folder into itself, iCloud may do the same on a Mac, or a settings reset pointed BookyAI at a new, empty folder. Your books are still on the disk. From version 1.9.22, an empty Library looks for them: when it finds books in another folder, it says "Your books are in another folder" and lists the folders it found, each with "Use this folder". Click it and your books are back. "Choose another folder" lets you pick the folder yourself. Your research and settings are not affected.