There are no layers. See the screen shot below.
I seem to have fixed it by removing that layout and making a new one.
That was because you were in Local View
Say what now?
In the menu View -> View Global/Local
or press the numpad / key as a shortcut
This is enough to switch between global (that is default) and local view.
the main interest of local view is if you have a lot of object in the viewport, you can select one and switch to local view, it will only display that selected object.