I have a nectarine tree that is 3 years old. I live in Southern CA on the coast. This last winter we had a few days of real cold weather. My tree has only 1 living branch.
How Do I Force My Nectarine Tree to Produce New Branches?
I don't think it would get cold enough in southern CA to harm a nectarine, so I suspect something else is going on. You need to think about the general care of the tree, especially soil, water, minerals, and light. This article should get you started: https://www.gardeningknowhow.com/edible/fruits/nectarines/growing-nectarine-fruit-trees.htm
You also need to watch for the common diseases and pests (nectarines are a type of peach, so this article on peach disease will tell you what to look for): https://www.gardeningknowhow.com/edible/fruits/peach/common-peach-diseases.htm
Finally, here's some advice on pruning - look through to the bottom of the article: https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-thinning.htm