I had this problem a while ago and now especially frequently in my new project after getting my full C2 license.
While autosaving, the Construct 2 sometimes suddenly moves to the autosave file instead of the actual project.
This results in saves not being made in the project and prevents you from previewing /debugging your game.
It's easily solved by
->save as new file
->either make new file or just replace your project file.
I think it's in that one second after the autosave is done, when I clicked on something or when I pressed preview right in that moment. I am not sure though.