These cameras are complex these days. You should give us a rundown of what you've tried to far.
When the AF-Assist light doesn't come on when you think it should, it's for one of two reasons. Either you have AF-C selected, or the focus point isn't centered. Page 210 of the English D7000 manual lists the conditions needed for AF-Assist to work.
An off-center focus point will cause this problem if there's nothing under the focus point to lock onto. So my suggestion is to check your selected focus point.