Master is damaged. You'll need to rebuild the system databases, then once SQL starts restore your backups of the system databases to get all your logins and other stuff back.
http://www.simple-talk.com/sql/backup-and-recovery/the-sql-server-instance-that-will-not-start/
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability