I'm running ArcGIS Pro 3.6.1 and this just happened to me. I was giving a live demo when the Map suddenly stopped displaying the layers that I was turning on via the checkbox in the Contents pane. I had Uber hexagons displaying count data with labels. Layers had displayed correctly just minutes before the meeting and then . . . nothing.
When I restarted ArcGIS Pro and loaded the aprx, I got the dreaded "Failed to open map view". I am not able to copy/paste or duplicate the offending Map, but the other 2 Maps in the project still open.
I ended up rebuilding the project on the fly to be able to do the demo, but that was far from ideal because some layers that had been based on selection logic involving multiple steps could not be done that quickly.
This software behavior is totally unacceptable and needs to be fixed ASAP. Oddly, there was also no "backup" copy of the project file. While I didn't lose any feature classes, there is a ton of work that goes into symbology, label expressions, definition queries, selection map layers, etc. To lose several hours of work is very sad.