Arun Sirpal had gremlins infest his server, causing a service pack installation to go sideways:
I try to start the service but it fails. (I was desperate ok!)
Script level upgrade for database ‘master’ failed because upgrade step ‘msdb110_upgrade.sql’ encountered error 5846, state 1, severity 16. This is a serious error condition which might interfere with regular operation and the database will be taken offline. If the error happened during upgrade of the ‘master’ database, it will prevent the entire SQL Server instance from starting. Examine the previous errorlog entries for errors, take the appropriate corrective actions and re-start the database so that the script upgrade steps run to completion.
Cannot recover the master database. SQL Server is unable to run. Restore master from a full backup, repair it, or rebuild it. For more information about how to rebuild the master database, see SQL Server Books Online.
To be honest, rebuilding master would be my last option.
Read the whole thing, and then double-check that you have good copies of master & msdb somewhere.