Suddenly, all files uploaded are 0 bytes -- totally baffling!

I maintain a telephone directory where each letter gets a web page. For years, I have uploaded changes to a server without incident.

I recently made a change on a couple of letters and uploaded those to files to the server. Both files appear as 0 bytes.

Figuring that maybe I did something wrong and introduces an error when I changed those two files, I decided to do a test. I uploaded another file (letter Z) without making any changes. In other words, I was uploading the exact same file that had been successfully uploaded months earlier. But now it, too, has zero bytes and can not be displayed.

I really have no idea where to start because I'm totally baffled. Anyone have an idea?

Unrelated to SmartFTP. This is how my server reacts when it is out of space. Sorry for the false alarm.