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

When a connection attempt is rejected because of an authentication failure that involves a bad password or user name, a message similar to the following is returned to the client: 

"Login failed for user ''. (Microsoft SQL Server, Error: 18456)".

Additional information returned to the client includes the following:
"Login failed for user ''. (.Net SqlClient Data Provider)" ... and so on

Solution:

• If you are trying to connect using SQL Server Authentication, verify that SQL Server is configured in Mixed Authentication Mode.
• Run the SQL Server Management Studio as an administrator.
• Right click the root node in the object explorer and select Properties.
• In properties window, select Security.
• In security tab, select the SQL Server and Windows Authentication mode and click OK.
• After that, restart the SQL Server service and restart Management Studio.
• If you are trying to connect using SQL Server Authentication, verify that SQL Server login exists and that you have spelled it properly.
• If you are trying to connect using Windows Authentication, verify that you are properly logged into the correct domain.
• If your error indicates state 1, contact your SQL Server administrator.