Ftp fails to connect if you are not using the correct username and password. Sometimes you may have a Force Password Change enabled on the server due to security that your password will be expiring time after time. You can reset the password to try again as your password may not be the same forever.
Login in to the control panel will verify if the username and password are correct if you are using Cpanel credentials. Verify the FTP credentials as well if you are using a specific FTP account.
You can also reset the FTP password by clicking on the change password tab.
In case you have forgotten your FTP credentials, you can click on configure FTP client to see the username and the server name which is your FTP hostname.
You may have a brute force issue, it is a firewall that senses some un-trusted login attempts and blocked the connection IP. You can use a different connection to test that and ask for support to white-list the IP to solve such a problem.
Note that you can use the server IP as a hostname for the FTP connection.