Solidarity with you all! With this being such a serious issue, I'm surprised/disappointed that Esri has not chimed in. Maps should not spontaneously become corrupt.
I've had this issue with maps in several different projects, and it's usually just one map in the project that has the issue while the others open as normal. I have not found a way to recover any of the corrupted maps.
I tried the copy/paste suggestion, but "paste" is grayed out.
I tried restarting my computer, and tried opening the project on a different computer. Neither option was successful.