Why my project is emptying after restart an editor?

Hey guys! Please can you help me
I’m just start learning a UE4 and use for this tutorial series…
While I work in the Editor, everything is fine. But, when im close Editor and open my project at next morning - all object is dissappeared, Im have a black screen and no one object in World Outliner…
When I’m use some solutions from tutorial in other project (what saved and re-open was successfull) this project buged too. Save-close-open and just black screen… Here we go:

Whats wrong?
Thank you.