To fix this issue run

CREATE LOGIN [BUILTIN\Administrators] FROM WINDOWS

go

sp_addsrvrolemember

@LogiName='BUILTIN\Administrators',

@RoleName='sysadmin'

Or you can also take the help of this link http://www.sqlserverlogexplorer.com/fix-microsoft-sql-server-login-failed-error-18456/[/url]