In this article I will show you how to change the authentication mode to SQL Server and Windows Authenication.
As I mentioned in 18456 Error article enabling SQL Authentication works in most cases where SQL Server has just been installed or this option was never enabled.
There is one step that is trickly which is Microsoft SQL Server User Interface (UI) bug but I will talk about it when come to this step.
Using SSMS connect to your instance using windows account (you need to have permissions).
Right click instance name and click properties.
Server Properties window will appear. Go to Security Section.
See below screenshot that might be causing SQL login to fail
You should set Server Authentication to SQL Server Windows Authentication Mode (see below). Once you select SQL Server Authentication you have to restart the server.
Restart required: You have to restart the service when you change server authentication but bear in mind if someone hasn't restart the service this might be the reason why you get 18456. Remember service restart should be carefully planned on production servers as it will clear certain information (cache) and may impact performance of the server.
NOTE: Once upon a time I tried to restart service using SSMS 2012 on Windows 7 and it didn't work.... actually the message below was hidden behind SSMS window and I just did not see it so ensure you get this message when you decide to restart the service.
Hope that helps