SQL Server Agent cannot be started

Hi,

The SQL Server Agent cannot be started after installing sql server patch 11.0.6544.

I've check the SQLAGENT.OUT and the actual error is:
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2016-10-05 10:02:39 - ? [100] Microsoft SQLServerAgent version 11.0.6544.0 (X64 unicode retail build) : Process ID 1880
2016-10-05 10:02:39 - ? [495] The SQL Server Agent startup service account is S901\S901_SQL.
2016-10-05 10:02:39 - ! [150] SQL Server does not accept the connection (error: 18456). Waiting for Sql Server to allow connections. Operation attempted was: Verify Connection On Start.
2016-10-05 10:02:39 - ! [000] Unable to connect to server '(local)'; SQLServerAgent cannot start
2016-10-05 10:02:39 - ! [298] SQLServer Error: 18456, Login failed for user 'S901\S901_SQL'. [SQLSTATE 28000] 
2016-10-05 10:02:39 - ! [382] Logon to server '(local)' failed (DisableAgentXPs)
2016-10-05 10:02:39 - ? [098] SQLServerAgent terminated (normally)
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Looks like the local user "S901_SQL" failed to login but I haven't change the password or any setting of this user.

Also the SQL Server (MSSQLSERVER) service can still be started which is also Log On As the same user: S901_SQL.

May I have the solution asap please? As this affect our production environment!! Thanks!


Following are my environment information:
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OS: Windows server 2012 Standard
DB version: SQL Server 2012 Enterprise 
Server name: S901
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