My tabs reset to like five years ago.

I was on Safari for a bit, got out of it for a bit, came back and my Safari had basically reset itself to the tabs from when I got my current iPhone. All my Private tabs were gone and some of my tabs in the regular mode were gone and some old tabs that had been gone for years came back.

I still had my current History and suggestions, but I also lost everything in my Reading List too.

What would cause a reset like this?