If you are unable to connect using FTP client, please try the below:

1. Check if the hostname, username and password are correct. Username and password are case sensitive.
2. If you are not sure of the username and password, you can re-create the username or change the password from your Plesk control panel.

At times, it could be a line disruption and should be back to normal in sometime. Please try connecting at your convenience after a few minutes.