When using a photographic or drawn reference to base your models on, it's a common practice to load the reference image in the background of the 3D View (see Chapter 5). However, when working this way, you may orbit your view to do a spot-check and then when you return to side (or front or top or camera and so on) view, the background image may disappear, even though the Background Images panel in the 3D View's Properties region says it's still there.
The answer here is that you're viewing the scene through Perspective view rather than the Orthographic one. Blender doesn't show the background reference image in Perspective view. Switch back to Orthographic by pressing Numpad 5. It makes sense to use Orthographic for reference images because a Perspective view introduces distortion and scaling to the way the scene is viewed, so it wouldn't be a good idea to model from reference in this type of view even if you could. The Orthographic view is much more effective at getting a model to match a reference image.