This question already has an answer here:
For the last month or so, seemingly at random and with no discernible pattern, my menu disappears. However, when I look at the Dashboard, what I find is that all the menu items have been deleted but the menu itself is still there. I can select and add the items back and they reappear. There is nothing in my error logs, and I'd guess it's happening about once/week, though that's just a guess.
I am using a modified default theme, though the theme has not changed for over a year and this problem started recently.
Because I cannot predict when this will happen it's difficult for me to remove all plugins indefinitely while I wait as it will break lots of functionality on my site. However, none of the plugins are directly related to the menu system. And they've all been in place for quite a while.
I've seen some posts that seem related to this problem but they all involve huge menus. My menu only has 4 items! The items all point to Pages. The structure/contents of the menu have not changed for a couple years.
Any ideas? Even ideas on how to troubleshoot this would be appreciated.