This issue has been closed as 'Won't Fix' due to an extended period of time without updates. If this issue is important to you please let us know by posting on the AnswerHub or UDN, and Epic will re-open the ticket for further review.
It's been reported that maps are going missing from projects and while they're showing up within the Windows Explorer folders, they're not showing up within UE4. You cannot migrate them in, import them or even copy and paste the maps folder to a new project.
I have tried opening this myself, as the project is included in the AnswerHub post. I also tried moving the folder to a 4.8 project and that did not work either.
There are no known repro steps.
Edit MattK: This is likely due to versioning issues where the map is newer than the engine version or something similar.
Head over to the existing Questions & Answers thread and let us know what's up.