How to: Restore the Service Master Key
The service master key is the root of the SQL Server encryption hierarchy. It directly or indirectly secures all other keys in the server instance. For this reason, the service master key is the most important key in the server. It is unlikely that you will ever need to restore the service master key. In you do, you should proceed with extreme caution. For more information, see RESTORE SERVICE MASTER KEY (Transact-SQL).
Copy the backed-up service master key from the backup medium to a directory on the local file system.
Execute the following Transact-SQL command:
RESTORE SERVICE MASTER KEY FROM FILE = '<complete path and file name>' DECRYPTION BY PASSWORD = '<password>' ; GO
Where password is the password that was used to encrypt the key for offline storage.