Login failed for user 'domain\username'. (Microsoft SQL Server, Error: 18456)

Scenario - I was connecting my local SQL server by using SQL authentication. I got an error code: 18456. After digging a bit i found the solution - SQL server error: 18456 overview - This error occurs when your authentication request is been successfully accepted by SQL server named, but due to some reason SQL server is not able to grant the access to connect.

Reason - This error reveals that SQL server authentication is not enabled in local server.

To enable SQL server authentication, Follow the below steps

Step 1 - Go to SQL server properties by Right click -> Properties.

Step 2 - Enable SQL server authentication mode.

Step 3 - Restart the SQL server.

I hope it will help, Check here for more detail

Solution by Pinal Dave:

• Create new user with Administrator privilege with same username and password as of SQL Server 2008 in operating system.
• On SQL Server database create new user by expanding DatabaseNode >> Security >> Login >> Create New User and add this new user with Windows Authentication radio button selected. This user can be only added by selected Windows Authentication it is Operating system’s User Login. Once above.you should perform the above steps using windows authentication

Also see http://bjtechnews.wordpress.com/2012/03/20/microsoft-sql-server-error-18456-login-failed-for-user/