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