How to put Nikon 18-55mm lens onto D40 body?

I've done it before a million times but this time for some reason it just won't click into place. I've followed the manual instructions but no joy.
Can someone please tell me how to do it.

Can you send us close-up photos of the rear of the lens and the mount on the camera body?

Is the lens working on the camera? Can you remove the lens without pressing the lens release button?
Try mounting another lens on your camera and also the problem lens on another body to confirm where the problem lies.

Line up the dot on the lens with the dot on the camera body by the lens mount and make sure they're seated firmly. Turn the lens counter clockwise (as you look down the lens towards the body) to lock it.