All Avatars Disappeared from the Entire Site

Since about 7am Mountain time today, all avatars disappeared from my busy site. They don’t show up on the front page, but don’t show up in the admin backend either. I get a lot of activity and a new comment is made about every 3 minutes.

This happened without me touching anything – I have not done any updates, upgrades, modifications to anything whatsoever. It just happened completely on its own. After I had it confirmed from my members that it’s not just my browser, I reinstalled WP (v3.9) but it did not fix the problem.

I also use Simple Local Avatars plugin to give my registered members the ability to upload local avatars without the use of a third party service, so I also tried disabling the plugin (so Gravatars should show), but avatars still don’t show up.

I use Twenty Eleven theme with a few simple mods, but have not made any tweaks to it in months (during which time everything worked).

I also use W3TC but there has also not been any updated in a few weeks. I emptied all caches but the problem persists.

What happened to my site? Can anybody help?