Nikon D80

i had the same problem occur a couple of times. i found that removing and reinserting the lense cured it!

EXPLANATION.
1. The problem with 18-135 lens is because it make from plastic. Main problem from plastic is it can be reshaped after a few usage. When the shape ran out, the contact point will be defective. When the contact point didnt really touched, the processor cant send signal to lens to do autofocus (the lens focus using the lens motor, not the body). When u set to manual focus, the body doesnt need to instruct the lens to focus.
2. You are focusing something that is to near than the focal point.

SOLUTION.
1.Try cleaning the contact point on the lens. Then mount it back until the F showing up (F= aperture). This solution depend on your luck too. if the gap between to contact is so far, then it might happen back. Usually when u need it badly. haha
2. try to get more distance from your subject. Just keep press the shutter until the camera focus.

CONCLUSION
1.But new lens with metal mounting. It will last and durable.
2.Buy macro lens to close up. normal lens wont focus at short focal point!!~

I had a issue like this..
the service person in the Nikon told that the lens had problem with alignment !
so i gave it for service !!

The camera shutter will not work until the green focus light is fully illuminated in the view finder (green dot on the extreme left).  Auto focus sometimes goes into a "search" mode which does not necessarily end up "in tack sharp focus."   Also, depending on the lens you are using, there is a minimum focus distance at which you are limited for focus for the lens.  Check the lens specs.
Good luck.