Problem: On a new installation of SQL Server Express, I changed the password, enabled the sa account, but still got an error when trying to log in - "login failed for user sa"
Solution: Verify SQL Server has been configured to allow Mixed Authentication Mode
Reference: https://support.microsoft.com/en-au/help/555332/-login-failed-for-user-error-message-when-you-log-on-to-sql-server