Layers are wonderful. Sometimes, though, when you don’t quite understand what you are doing (and sometimes, even when you do!), you will accidentally delete them all by creating a single layer of them. This is called flattening the image and it results in a single layer in the image. If you were lucky enough to only have a line drawing in the image, you can usually get your layers back by selecting the background of the image with the Magic Wand tool, reversing the selection and then getting each object into its own layer.
One reason to do this is so you can colour your line drawing and add a different colour to each object. Take a look at this short video to see how.
–Sherry aka the prancing pixel