i guessing you've put the walls, floor and roof in the render layer you're doing an AO pass on, so the reason why it would be all black is because you've closed the room off so the walls, floor and ceiling have occluded the light from getting in
if you put in windows or remove the roof or a wall then everthing will show up
of course if you haven't then i dont know what you've done wrong!
that's a "Ch" pronounced as a "K"
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