First of all, please make sure that you are using the correct FTP details (such as hostname, username and password). You can find these details in your members area > File Upload Details page.
If you are using your own domain, please make sure that it is pointing to our nameservers (the propagation takes 24-48 hours after nameserver update at the domain registrar). In this case you can use our server IP address as your FTP hostname. The IP address can be found in the members area > File Upload Details page.
If your domain is already resolving to our server and FTP connection still fails, please try to use the cPanel -> File Manager tool to upload your files or contact our support.