Library menu missing at top right of Lightroom screen.

I'm using Lightroom Classic. Have an extensive (20K+ library). Did some edits yesterday, shut down, said OK to do the weekly backup. Started today, found that I no longer had a Library! The main menu list across top right used to start with Library. Now, it starts with Develop. No Library option there at all. And, at top left, my Navigation menu had just the single item I'd entered (via digital camera capture) before noticing Library was missing. I saw no mention in my google and forum search about anyone else ever having this problem. How do I resurrect my years of work? Yesterday's .zip backup is present, but I don't want to touch it until I'm sure it'll be safe.