That doesn't look like any objects have disappeared at all.. it simply looks like the ground polys are drawing on top of most of the other objects, which is an alpha problem.. when any polys have an alpha channel in their material texture OR their vertex colors, this is exactly the effect you get. you can clearly see that the objects are still visible, but being overwritten by the ground if you look at the ramp, or those "hutches" in the upper-left.
what I suggest is that you apply the default lambert1 (assuming you haven't modified it) to EVERY object in the scene. If it still draws the polys in the wrong order then you have some alpha somewhere in your vertex colours.. if not, undo, (or re-load the scene) so your textured materials are restored then remove all vertex colour from every object.. if it still draws wrong you have an alpha channel in one of your materials textures (b.t.w, it doesn't matter if the alpha channel is fully opaque or not, if it exists at all Maya will draw the polygon order incorrectly)
if neither of these solve the problem maybe your video card/drivers are dodgy..?
:nathaN