I'm guessing that the artist accidentally deleted the layer?
Another possibility is depending on the type of layers, there are a lot of features CS2020 have that CS5 don't and most likely won't show because it's coming from a newer version. For example, if I use a color lookup layer (which debut in CS6), it won't show on CS5 because that feature isn't available. I do not know what kind of layer the artist used, but if it's a regular layer, then it should show like all the others.
Those are my two guesses and I have a feeling it's the latter.
That's the least you can expect when you open new files in 10 year old software. For most other applications (InDesign, Illustrator), you wouldn't be able to open the file at all. Photoshop is particularly forgiving in this regard, but all new features over ten years will naturally go missing.