There is no way in the world I would have figured that out. I haven't done anything remotely like debugging in 1-2 decades and even that was limited. I'm not sure why your coworkers think it's appropriate to require that every user be a computer programmer.
As for how it got broken? Not a clue. I haven't used FTP in years and, while I have a copy of Dreamweaver on my old computer, I haven't used it in a while and I haven't even logged on to this site for a very long time. Literally everything I've done on this site in years has been through the Wordpress interface (with the possible exception of uploading a graphic or text file at some point, which would be to outside the WP folder, and ages ago).
You say "Turning on the WP_DEBUG option in the wp_options.php..." but I couldn't figure out how to do that without access to the WP admin page. I read the debugging link and it made no sense to me (it seemed to be for people with access to the admin page). I have a lot more computer skills than the average internet user but it was instructions that required tools and background knowledge I don't have. I certainly have never deleted or changed any of those files via FTP or Telnet or in any other way. If it was hacked, then I reported that to Sonic at the time (some of my sites have been hacked here and there but I can't recall if this one was, that's why I'm careful to keep my site updated) and Sonic would have done what was necessary to fix it.
I was able to get into the site today as both a user and an administrator. I've exported and downloaded the site content (as well as for my other sites) which will give me some breathing room as I search for a new hosting site that treats customers like Sonic used to.
Thanks again! You are the personification of the Sonic that I remember from decades past.