There are some programs that use PNGs as their native files and you can use layer in them like Fireworks (which is also from Adobe now) but Photoshop doesn't support Layers in PNG files and will only open it as a 'flat' unlayered image. It does keep any transparency information from the PNG though so that's a bonus.

I think that Adobe wants to keep the Fireworks product going and if Photoshop could open up these layered PNG files and keep all the options then Fireworks would cease it's usefulness.

Are the PNG files you're trying to open Layered PNGs (and so Photoshop is opening them flattened)? Or is Photoshop not opening them at all?