Half my project isn't showing up inside Unreal, what can I do?

The files are still there. I can find my assets and my maps and whatever else there might be in Finder, but not inside Unreal Engine’s content browser. Through file > recent I saw a missing level, but opening it errors out with:

Failed to open map file. This is most likely because the map was saved with a newer version of the engine.

I have backups, but I don’t want to roll back a lot of work on my personal project. It would also be nice to know how or why this happened, as I haven’t changed versions. Lots of hours of googling frustration later, I’m hoping this can be solved.

Some of the assets Unreal Engine does see can be opened, some not. The latest assets I’ve made do nothing when doubleclicking on them, and rightclicking shows this error:
The message log itself has no errors though.

EDIT: The missing blueprints make this entire project completely unsalvageable. If only there was a force rediscover content-button.