I just got home a little while ago from Tampa and found all of this going on. Apparently somehow the index.html files in all of my domains on the server got replaced by this single file that displayed that message. I have no idea how that happened, but I contacted my server host and they did a security update on the server.
It was really no big deal, since all of the other files were intact, and I was able to replace the files pretty quickly. Nothing else got compromised or damaged, apparently.