Most likely it will not grow back. There is some good news though. There are products that you can buy or procedures that you can get done that grow your hair back, stop thinning or transplant hairs in your bald spots.
It depends why it is happening. As we age, our hair can thin and there is not too much a person can do about it. If it is a too tight pony or because you wear wigs too often, you can do something about that.