Try taking the lens off and cleaning the contacts on both the lens and the camera body. Make sure the lens setting is set to A and the camera setting next to the lens is set to AF Then look at the display on the top of the camera and make sure it says AF-A.
If it still doesn't work, try a different lens. If that makes it work you have a bad lens and you can send it in for repairs or replace it.
Thomas Falater