Solution to a similar problem in case someone googles this like me can also be that the map accidently got closed, you have to find it in the Content Browser (orange icon with models on a plane situation on it) and double-click it to reopen.
If you have the Starter Content folder like me, you might have built on one of those maps, just go into the folder (in Unreal Engine) and then to Maps.
I moved around the Starter Content folder so my map got closed, everything black, World Outliner empty, Content Browser still had my things though, I was really confused.