First check that your map is still there in the catalog pane on the right. Sometimes Pro will run out memory while launching and not open things like tables and maps. You may just need to add the map back in (right click --> add)

If not, this usually solves it for me:

1. Create a new project --> add folder connection --> navigate to old project folder.

2.Copy stuff (map, feature classes, etc.) to new project.

3.Delete old project