you copied the entire Calibre Library folder
into Dropbox, then all you need to do is in Calibre press the menu/toolbar item that looks like a group of books and tell Calibre to use the library folder in Dropbox. If
you moved the book folders from within
the Calibre Library folder, you want to move them back, then press the Library menu button and tell Calibre to move the Library into Dropbox. My understanding is that you DO NOT
want to mess with things inside
the Calibre Library folder.
Hope this helps!