There are some times master database needs to rebuild due to corrupt or some other issues. In real world every day your backup jobs will be running and do backups for your application databases along with system databases..
Here are the steps you can follow how you can rebuild the master database.
1) find setup C:\Program Files\Microsoft SQL Server\100\Setup Bootstrap\Release. and run the following command
setup /ACTION=REBUILDDATABASE /QUIET /INSTANCENAME=<instance name> /SQLSYSADMINACCOUNTS=<accounts>
once the command is executed and message will display databases are rebuild.
2) Now restore good backups of master,model,msdb databases in order to run properly according to your sql server original configurations.