I can't get my lens to focus on Nikon DSLR?

I have a Nikon D100 camera and a Nikon AF Nikkor. 70-300mm. 1:4-5.6G telephoto lens.between 70 and approx. 120mm the lens will focus both manually and automatically.once i get past 120mm the lens will no longer focus.
i have tried cleaning the contacts, changing the lighting, my distance from focal point, resetting the camera back to default all to no avail.
what can i do to get the camera to focus when zoomed past 120mm!

It sounds like the lens is damaged.

Somewhere along the life of the lens, it was dropped or knocked into something which caused the lens to no longer auto-focus when it is in the 120 mm range or longer.

The lens needs to be sent in for repair

That's not normal. The lens is broken. Ask the store you bought the lens from. They should point you to their authorized service center or accept the lens and process the repair themselves.