This typically indicates that files in your database are corrupted and can no longer be opened. The most common reason for this is storage of databases on network drives, flash drives or syncing programs like Dropbox where the connection to the database is not fast or reliable enough to write the files correctly. It is normally not possible to recover files damaged in this way, but you can prevent further problems by moving your database to a local drive, and recreating the database to rid it of errors.
See also “Can I store my database on an external drive or a network drive?” and “How do I recreate my local database if it has become corrupted?” under the Databases category.