Hi, I have sitution where my Database has corrupted but some how we have recovered it but while restoring the .bak file it shows an error
The error message indicates that the backup file is corrupt (something happened after a successful backup to break the file), incorrectly formed (possibly caused by a backup that failed mid way), or possibly not even a SQL Server backup (someone just named the file with a BAK extension).

When attempting to restore a backup from Management Studio Studio, SSMS uses the RESTORE HEADERONLY command before it actually restores the database. This step is the step that is being reported as failing in your error message. 

This tells us two important things:


The operation fails prior to the actual restore.
At minimum, the backup header is corrupt or missing, but possibly more.


The header tells SQL Server information about this backup, as well as the database from which this backup was created. Without this information, the backup cannot be restored.

Your best bet is to move on to an older backup and attempt to restore that. If you have no good backups, then you're pretty much out of luck.