I have a normal map looks like this:
I switched tabs back and forth a few times doing normal editing and fiddling stuff and end up with this:
I was able to exit without saving, reopen it (the layout was good), then do a save-as and work on that version. After a while the save-as'd version died as well. This is happened once before (long ago, would have been under 2.9-ish) but it was just a simple map so I gritted my teeth and made a new project.
The layers are a mix of shape files and enterprise and file geodb FCs, nothing fancy. Background is (was) one of the stock Esri ones.
This was on the 10th revision of this map, maybe the Project just went corrupt like the old MXDs used to?