Library and Folders out of sync

I keep my calibre library in dropbox to keep it synced across my various PCs and add books from whichever device is convenient. I did something to get them out of sync and now some books are no longer in my library but the books, covers and metadata files are still in the folder structure.

Is there an easy way to re-add these books into my library? I tried using Library Maintenance --> Restore database but that didn't work.

Thanks in advance!