how do i rebuild the master database, i have no backup to restore from.
You can't rebuild it. If it's corrupted you can try to fix the corruption. I'd strong recommend reading Paul Randal's blog for details on this issue.
If it's been deleted somehow... it's gone. If you have your database files for the user databases you can attach them to regain access.
Beyond that, as Fatherjack says, a lot more detail is needed to understand the problem.
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