I setup a new SQL Express instance on my local machine. When I tried to complete the install, I kept getting the error message .Net SqlClient Data Provider Class: 14 Number: 18456 Message: Sql login failed
After checking the credentials, and making sure the user account had proper permissions, I was stumped.
What I forgot to check was that SQL was setup for mixed authenitcation. It defaults to Windows authentication. Once I switched to mixed (Windows and SQL logins) then it worked fine. (Right click SQL instance in SSMS > Properties > Security > Server authentication)