The SQL database can get inaccessible or hidden due to several reasons such as insufficient available space, damage/invalid database file header, power failure, Virus attack, corruption of media/driver, sudden application shutdown and insufficient available space, then you can fix damage SQL database issue by using
Recoveryfix for SQL Database recovery application to repair MDF files and restore complete SQL database.
You can try this tool from here: http://www.en.repairsqlserver.net/