SQL Server 2014 login issue
This always happens because during setup you forgot to adjust your instance to be in both Windows and server authentication. Open SQL Server Management Studio; log in using Windows authentication; right click on your server  then properties, then security tab, and mark on both Windows and server authentication; then click ok.

Close SSMS then here is the trick:


start SQL Server configuration
stop all the services
restart all the services


then you will be able to connect in SQL Server authentication.