Originally Posted by
DareDevil
I'm assuming your on AF (Autofocus)
The lens can't find an object to focus on.
Switch it to MF. Focus it yourself and you wont have that problem.
I won't get into anything more technical then that.
DD is exactly right. Check your focus indicator (Nikon's have a green dot in the viewfinder that comes on when focused) because your camera won't shoot in AF if it's not focused.