In case anyone else runs into this, I saw an issue in a couple of Moodle courses today where embedded YouTube videos were suddenly not displaying on pages in Moodle courses.
Jon Fila identified that there was a problem with the embed code, and then we figured out that the specific problem is that videos embedded with “http:” in their iframe embedding code no longer are working.
A quick fix for this is to switch to the code-viewing mode for the page content, find the URL inside the YouTube iframe code, then add an “s” to the end of the “http”. Save the page and the video should now display correctly.