All objects on base layer not showing up on layers

Thanks David. I went in to have a look at some of the layers on a few different slides, right clicked on slide, checked format background - and no solid fill. No image either ... is there another way to check for this?

The strange thing is that it has happened to all slides in my module with layers and each layer on a slide - including each individual screen-recording slide and other types of slides with buttons etc - so for example, on the screen recording slides when you get a tip - you see the tip caption on a white screen ... weird. See below in screenshot with a layer open... All base layer objects visible but nothing in them... and what you see on the slide is what you see in publish mode.