When I opened Unity for the first time, the Island demo showed up, and since then I have run through a few of the basic tutorials.
Now, though, when I open that project or any other - nothing (terrain, models, everything) is visible in scene or game view. I can add new objects and view them, however.
Is there some option I accidentally clicked or something to turn of seeing stuff? 
You don’t have any scene files open…open a scene file.