Hairfall due to straightening. It was?

Hi can u suggest a multivitamin that will help in promoting healthy thick hair that I had before chemically straightening my hair..my iron levels are 13 so doctor has prescribed an iron supplement and a hair growth supplement which is too expensive so can an alternate hair growth supplement be suggested ..I've also heard that multivitamins and bcomplex tablets will serve the same purpose and that hair vitamins are simply over priced.. A friend suggested me to use black seed oil .. What can I do to improve the health of my hair follicles and revive the damaged ones? Is it possible to bring the thickness of my hair back to how it was? Both my parents and their parents were blessed with thick hair n no signs of balding at all so that's not a possibility.. Iam confident that there's thinning of my new growth because the chemical for straightening was left on my scalp for too long..please suggest a remedy for strengthen roots and reviving damaged follicles