This is probably an exceptionally rare cause for this problem, and once spotted, it’s incredibly obvious, but since it took me about a half an hour to find it, I thought I’d post a note. It might just save someone some time.
I found that suddenly I had no content in the RSS feed for one of my WordPress based sites. The feed had been working but suddenly stopped. I searched for various causes of this problem and even uploaded my WordPress files (other than wp-content).
It turned out to be incredibly simple. I’d created a directory “feeds” which, of course, overrode whatever WordPress wanted to provide using that permalink.
I would imagine that I intended the “feeds” directory for one of the other sites on the same account, but the why is less important than the solution – delete the offending directory!