Go to Window > Layers Panel (F7).
JPG, PNG and bitmap images don't have layers. They are flattened to a single Background layer.
Photoshop PSD files support layers if the image contains any. Some people like to merge layers at completion to reduce file size. I always keep an un-merged version as a backup just in case I need to go back and make changes.
In this example, my PSD image contains many, many layers which I've organized into 3 groups called Text, Flames and Background. Clicking on the Background group reveals the layers inside it.
