default menu appeared

Woke up this morning and the menu locations were set to select a menu.
Thus every possible page and category was on the primary menu

The menu had been and should be set to Main Menu, the menu I created.

Simple enough I thought. I changed the location back to main menu and saved.

Well Main Menu is NOT the same menu anymore. The pages are still there from what I can tell, however the hierarchy is gone.
Instead of 5 top level pages with sub menus there are 2 with all the others under them.

I am just rebuilding from scratch.

My concern, is how do I figure out how this happened?
How can I keep it from happening again?
Is there a way to restore just the menus, should it happen again.
Can I backup my menus?

I have a database backup, but we added several pages yesterday and if I restore the backup all that work will be gone.
less work to redo the menus.

Can you images the trouble I would be in if this was a live making money site?

Here is a screen shot of my plugins.
https://www.diigo.com/item/image/3nvao/0ygd

This site has been a project now for two years, so lets not judge the site or the old plugins that might need to go.

thank you in advance for you serious comments and assistance.