the restore_database uses metadata backups Calibre creates by itself, not ones it requires you to create. Did you remember to include --really-do-it? The command you need to run is, in full
calibredb restore_database --really-do-it
You could use the Synctoy backup you have, as well, but you'd of course lose any changes you made since then. Also, you need a backup of both the directory structure and the metadata.db file taken at the same time.