When you restore a user database from a backup, that database must not be in use because any data in the specified database is replaced by the restored data.

If you plan to restore master database , you have more things to do before restoration. 

• Before you restore the master database, you may need to rebuild it from Microsoft SQL Server  using the Microsoft SQL Server Rebuild Master utility. 

Note : MSDE does not contain the Rebuild Master Database utility. To recreate the master and model databases if you are using MSDE, you must reinstall your MSDE-based application, and then restore the appropriate online backups.

• Restoring the master database requires that the restore operation have exclusive access to the SQL Server instance. To ensure this, you must start Microsoft SQL Server in single-user mode and restore the master  database from the most recent backup. To start Microsoft SQL Server in single-user mode , Please click here .
  
• When you finish restoring the master database, you must return Microsoft SQL Server service to normal (multi-user) mode .