Are you pressing any function keys? The F2 key toggles the Layers palette on and off.
Have you selected a different tab in a palette group? The Layers and History palettes are usually grouped.
Have you minimized the Layers palette? There's a triangle next to the palette name that will collapse and expand it.
Is the palette hidden behind another palette? You might try closing all the palettes and opening them one by one (done from the Window menu).