When a USB which contains the database is removed from a computer while changes haven’t been completely saved, the database is damaged and cannot be opened. In this case the repair functionality can help by repairing KeePass database file from tools menu.
In case the user forgets or loses the Master Password, the repair functionality won’t help.
In case the header of the database, which is the first few bytes, is corrupted, again the repair functionality won’t help.
To avoid this kind of situations, backups can be done regularly.