Rendering Nothing But Black

[hellopaul]

As the topic says, I have a scene which renders (instantly) just black....or whatever colour I have the background colour set to in the camera's attributes. I have lights, I have tried creating new render layers, everything has proper materials, nothing is ludicrously huge or tiny, there are no file references, the camera's clipping planes are properly set, I've tried tweaking the render settings, I've created a new camera, new lights...but nothing...
EXCEPT...
If I chuck in a few polygon primitives, they render perfectly. Just a polygon cube and a cone, all on their own, in glorious grey Lambert.

If I create a new scene, it renders perfectly. If I open a completely different scene it renders perfectly.

Any ideas?
Maya software renderer on Maya 2009 64-bit (I also tried the scene in my 32-bit version, same result) (Windows).

Thanks in advance!!

UPDATE:
I looked at the render diagnostics and found:
- polySurfaceShape41Orig has no shading group associated with it. This
means that it will not be rendered. Please assign it to a shading group.
- polySurfaceShape42Orig has no shading group associated with it. This
means that it will not be rendered. Please assign it to a shading group.
- polySurfaceShape43Orig has no shading group associated with it. This
means that it will not be rendered. Please assign it to a shading group.
- polySurfaceShape44Orig has no shading group associated with it. This
means that it will not be rendered. Please assign it to a shading group.

.....this went on for many lines.

I checked the materials, and they don't seem to exist in the hypershade. There are loads of shading groups, which I can graph to show the associated materials...But there's nothing under the materials tab of hypershade. Any ideas??

Thanks!

[akjones]

The shapes that have Orig in their name are probably just shape nodes that don't even render because they are intermediates used for deformations. I doubt that this has anything to do with it. Does the scene render in hardware? mental ray?

[mrwolfy]

Are you sure your shaders are attached? Try assigning another shader, just to see if the surface renders.

[hellopaul]

Hardware render works ok, mental ray renders nothing except one object which has a material with an incandescence map attached. In the Hypershade, there are NO materials except the usual Lambert1, ShaderGlow1, PartcielCloud1. However, there ARE loads of Shading Groups. But if I select an object in the scene and right-click to Material Attributes, the material appears normally in the attributes window, and each material is connected to the input of a shading group. As mrwolfy suggested, it looks like I need to attach my shaders...maybe. How do I go about doing this? As far as I can tell, they are attached.

I created a poly cube in the scene, created a new Blinn material for it, and it renders ok...with the rest of the scene still non-existent. I've graphed the Blinn shader group in the hypershade, and it looks exactly the same as the other shader groups. The scene has about 40 materials (nearly all with image maps - and Maya knows where they are) so I don't want to have to recreate them all!