Consider a practical case, where you, as a database administrator, perform below steps:

1.You copy your database file when your database is alive.
2.Then, you update the table stored in the data file.
3.After this, you shutdown the database using 'shutdown immediate' command and change the datafile with its 'alive' copy.

After this, when you try to mount your Oracle database, it does not mount.

Cause:

The fundamental reason for unmountability of the database is corruption in datafile. To prevent the corruption of datafile, you should never follow the above steps sequentially.

Resolution:

To resolve datafile corruption and to mount your Oracle database, it is advisable to restore the database from a standby database. However, if no such database exists, then you will need to repair the database. To effectively do so, you will need to search for a commercial dbf recovery application that can repair your corrupted database.

A repair tool to recover Oracle database and to bring it back to a reusable state can be easily downloaded from the Internet. Such tools can Recover Oracle Database after any logical corruption scenario using powerful recovery algorithms. Moreover, these tools do not make any change in the original database, making them completely non-destructive in nature.