SQL Server 2014 Authentication
A way of knowing why is this error happening would be to check the SQL Server log and look for details into why is this happening:

Try to login again with SQL Server Authentication, after getting your Error 18456 change to Windows Authentication and in your Object Explorer go to: Management -> SQL Server Logs -> Current - XX/XX/XXXX XX:XX:XX
Now search for your newly logged error and identify the error State 

With this State number assest the nature of the error using the following table:



As you can see in this example I failed to login because my UserID is incorrect:



Also make sure you have hibrid authentication. Check the server properties from the Object Explorer and go to Security section and make sure Server Authentication is set to SQL Server and Windows Authentication mode

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