Make sure that you're using www.yourdomain.com as the hostname, along with yourdomain.com as the username (no www), and the correct password.
Please note, everything should be written in lower case unless your password contains capital letters.
However, if you're still struggling to login please contact our support team for further assistance.
Although it is not essential, if your FTP client produces FTP logs, these may also be useful in resolving the problem.