[Adoby]

If you are lucky there are updated opf-files together with the book-files. These opf-files contains backup metadata that calibre can use to reconstruct the database.

There are two possible methods:

1. Use the library maintenance tools to restore the database, as suggested by BetterRed above.

2. Create a new empty library with the same structure as the corrupt one (there is an option to du just that) and add the books in the old library to the new library, from subfolders, with one book per folder. Calibre will then scan the old library and add all the books, using the metadata in the opf-files. It can take a while, if the library is large...

I would try 2 first, even if it is slower, because it creates a whole new library and leaves the old library as it is. Just in case there are more problems. But it means that you afterwards have to move the library to make the online backup work again.

Most likely the culprit was the online backup software. Perhaps you have had the backup software working while you had calibre running? That may create either a corrupt backup or a corrupt calibre library.