425 Error. Could not open data connection to port

I was able to successfully create a db connection and FTP, where I was able to upload my entire site to my remote server. However, I then attempted to upload some minor changes but my database connection step had disappeared. I’ve attempted to re-establish the database connection with the same credentials I had before, but am now receiving the error: 425 Could not open data connection to port 51391: No route to host.

What is this error and why am I now receiving it after a formerly successful connection with the same credentials?