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.
In addition to the AF-Assist light settings, if the Sigma is older, it may not be compatible with newer Nikon's. Sigma will know which versions are and yours might need to go back to their service center to have is chip replaced. Sigma can do that quickly and does not charge for it. Stan St Petersburg Russia