Here is some more detail from the person who deleted the sprint:
"One of our testers had created two new issues and somehow attached them to your sprint. The confusing thing for me was that, when I moved those two issues out of your sprint into our sprint, your sprint title remained on my board, albeit empty. That made me think it had become part of our project. I then deleted your sprint from our board."
This resulted in my sprint being permanently deleted and issues going to the backlog. Note that the above user is not working on my project or sprint board.
How is it possible for a user who is not a named administrator or developer for the project for which a sprint was created to delete that sprint? And how can I prevent this from happening again? Note that under Project Settings/People I have specifically named only those working on the project and removed any group access. Clearly this does not stop others from causing problems.