Indesign can't select any object or layer

I've opened an Indesign file which I last worked on a month or so ago.

The artboard looks normal - however I can't select anything.

All of my separate layers are showing in the layers panel, however within them, there is nothing showing at all. E.g I have layers for guides, artwork etc. but they are all showing as empty.

However, when I HIDE each layer, they correctly disappear on the artboard.

So it's like a completely flat document but with nothing selectable whatsoever. Layers intact but no objects showing within them.