I’m still pretty new to Blender.
Something has happened so that one object is visible but no others are visible.
The other objects are still in the Scene window and I can select them there. The little eyes aren’t turned off in there. If I have an object selected other than the one visible one, I can’t switch to Edit Mode. Under the View menu, Show All Layers is greyed out, which I assume means I’m already showing all layers.
I’m using this to make models for Unity, and if I import the file the other objects appear in Unity. So I think that means it’s some sort of Blender interface thing that I accidentally activated.
For the mean time I’m reverting to an older version and losing a couple hours of work, but if it’s something I could do by accident, I hope someone with more experience will recognize what happened.