If you truly did nothing, the only possibilities are: 1. Caching (but I doubt it, if it was styling or a missing changed image, I'd say caching), 2. A minor update was pushed to your site but your dashboard isn't showing it (I doubt this too), 3. Your web hosting environment changed causing some error (I doubt it), 4. You were hacked (I doubt it because a hacker would mess up more than a button).

Going back to the cache possibility. You say you didn't change anything, but what about weeks ago? This is rare, but in the past I've made changes at a website and then didn't notice an issue right away because the old version was cached. Or, parts of the old version were cached.

Side question: Since you can insert an image into posts with the image icon on the editor's toolbar, what's the point of this feature? I guess it saves a few seconds of time. But, it's not a feature I'm worried about personally.

You've checked your browser's developer console for errors?