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bent aperture lever? All other lenses don't have one as they are af-s. But I don't know much, so take it for a guess.
All Nikon lenses have a mechanical aperture lever, except the PC-E lenses and the new 800mm f5.6.
If all lenses are doing the same thing, check the AI linkage on the camera body - it's a black plastic ring that slides around the lens mount opening, with a little protruding tab that's normally at the one o'clock position (as you look at the front of the camera). If this gets stuck (or broken) the camera can't tell what the aperture on the lens is set at with older lenses.
If only some lenses are doing it, check the lens aperture linkages on the camera body and the lenses as Mike suggested. You might also take a close look at the contact blocks on the lenses, to make sure they aren't cracked or broken.