How to login with Windows Authentication mode after domain name changes in SQL Server? I am using SQL Servre 2008 R2. I recently changed my domain name and after that when I try to login to SSMS using Windows authentication, I get an error 


  Login Failed For User UserName
  Microsoft Sql Server Error : 18456.


How to resolve it? Can somebody please help to get login in SSMS using Windows authentication mode again?

Thanks in advance
You should login with SQL authentication (with your sa account)
and run this query:

DROP LOGIN [OldDomain\UserName]
GO

CREATE LOGIN [NewDomain\UserName] FROM WINDOWS WITH DEFAULT_DATABASE=[master], DEFAULT_LANGUAGE=[us_english]
GO

ALTER SERVER ROLE [sysadmin] ADD MEMBER [NewDomain\UserName]
GO


You will then be able to login with your new domain name (be sure you change the domain name and username in the code).

If you need to recover your sa password you can read this article 
http://v-consult.be/2011/05/26/recover-sa-password-microsoft-sql-server-2008-r2/