I have a Nikon D300, and it keeps telling me my lens is not attached! Anyone?

I have a Nikon D300, and it keeps telling me my lens is not attached!

Just the one lens. Do all the lenses have this problem or just the one?

It could be that the contacts are dirty.

Are you getting the fEE error message? Something else?

We need more detail about your problem

What kind of lens?
Try removing the lens. Clean the contact points in your lens (they are the copper colored metal parts sticking out of the lens' mount). You can use a damp cloth, a damp swab, etc. Don't soak. You might want to also clean the place where the lens mounts onto your camera.
Properly attach lens.
Hopefully this helps.
If this happens to all your lenses then maybe you should have it checked.