Why are my embedded public reports suddenly asking people to log in?

I have two embedded publicly available reports on my web site. Twice now, one or both of them have suddenly changed from just being available to this message:

You have logged in as Public. You are not allowed to access this view.
Please click here to login as a different user.
Click here to go to your Home Page.

What's going on? Why did the visibility change without notice, and why are they now asking people to log in?

Thanks,

Alex