If you are not able to establish any connection through FTP:

Log into the control panel for the domain.

Reset your FTP users password and make sure the password
matches with the FTP client program you are using.

If you are still unable to connect please connect through your
Windows Explorer for PC or Safari for Mac.

Please use this format for FTP:
ftp://username:[email protected]

Substitute username with your FTP username.
Substitute password with your FTP password.
Substitute yourdomain.com with your Domain name or IP.

This will fix most connection issues with FTP users that were working before then stop.
If you are still unable to log into your FTP account a support ticket would need to be submitted.