A damaged master database is evident by the failure of the SQL Server to start, by segmentation faults or input/output errors or by a report from DBCC. An example of an error might be damage caused by media failure in the area in which master database is stored.
The procedure used to recover a damaged master database is different from the procedure used to recover user databases. If the master database becomes unusable, it must be restored from a previous dump. All changes made to the master database after the last dump are lost when the dump is reloaded and therefore must be reapplied.
It is strongly recommended that the master database be backed up each time it is changed. This is best accomplished by prohibiting the creation of user-defined objects in the master database and by being aware of the statements and system procedures, and the equivalent actions in SQL Server Management Studio, that modify it.
Refer to the documentation for Microsoft SQL Server for information on the proper way to restore the master databases.