For me, this happened due to a bad backup and move process. I have no idea what the root issue is other than, after a move to a new server it stopped showing the content in the back-end editor.
To fix this, I mostly just copy/pasted from the front end into the editor and saved.
I also noticed that my photo galleries were not showing up anymore, so if the post has a photo gallery these are the steps I did to make sure the new post had it:
1. Open the post on the front end and view source
2. Find the gallery in the source (search “gallery” until you see image links
3. Look for the attachement id in the HTML img tags, write them down
4. In your post add the following:
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