Content in master page not loading in front-end

I was fiddling around with layout widgets locally on a test page, and while testing my local build, everything in one placeholder in the page's master page (which I hadn't been working on) suddenly disappeared from the front-end, when I view the page ​in a browser. I checked the Javascript console and there's just no code where the placeholder is, nor are there any unusual errors I can detect.  There's a note about "XHR Loaded" but that seems to be a timing network thing. 

I put in more placeholder content in the master page, and that displays, so there doesn't seem to be anything wrong with the master page not displaying anything at all. However, no pages that connect to this master page will show the content that originally went missing.

The most unusual thing is that when I go into the editor, everything appears like nothing is wrong! Content placeholder is there, content is there, everything's fine. While absolutely nothing in the content placeholder loads in the browser.

What might be causing this problem?