Likely causes include:
SQL instance is not configured to allow SQL auth
the login and/or password is wrong
the login does not have access their default database
Check the SQL Server error log as that will have more detailed info on the cause of the login failure. Those are not returned to the client for security reasons. The error log can be viewed from SSMS (Management-->SQL Server Logs) or via a text editor. The current log file is named "errorlog" (no extension) and located under the installation folder Log subdirectory by default.