Lenses are not only precision optical units, but also complex mechanical devices using nearly silent motors to focus the glass elements inside the lens. Autofocus lenses receive electrical signals from the camera they are attached to adjust focus. The Nikon 70-300 is mid- to long-zoom telephoto lens ideal for sports, nature and landscape photography. There can be a number of reasons why a lens will not focus. Before you take it in to be serviced, there are a few things you can do at home that might fix the problem.
Install a fully charged battery into your camera. The motor in autofocus lenses draw power. If your camera battery is not charged enough, or is not holding a charge it might not be providing enough power to drive the lens.
Clean the electrical contacts on both the camera and the lens. These contacts carry the electrical signals the lens requires to change its focus. If you change your lens often, there is a chance it became dirty. Moisten a cotton swab with a small amount of rubbing alcohol. The swab should be damp but not dripping wet. And clean the contacts by brushing them with the swab. Remove any excess moisture with a clean microfiber lens cloth.
Set the external manual focus/autofocus slide switch on the lens to autofocus. Also check the focusing distance switch. Some longer telephoto lenses will have a focus range switch where the lens will focus from a minimum distance to infinity and have a selection where the lens will focus from 1.5 meters to infinity. Reducing the focusing distance allows the lens to perform quicker when shooting fast-moving subjects.
Set the camera to autofocus. Depending on the model of Nikon camera, it might have a function to turn off the autofocus.
Reset the camera to its factory settings. Your camera drives your lens, and you might have changed a custom function that is affecting the lens focus.
Clean the infrared focusing sensor on the camera. Autofocus systems use a beam to determine distance. On the front of your camera there will be a small sensor that might have a red or clear plastic cover, make sure this cover is clean and not obstructed.
Listen to your lens. Inside autofocus lenses are small gears and in some cases motors. When they move, they will make some noise. Some lenses are quieter than others, but at a minimum you should hear the glass moving. If you have checked all of these items and you still don't hear your lens move, you should take it to a Nikon authorised dealer for service.