Hair stops growing for many reasons, but one of the most common is the conversion of testosterone to the hormone DHT. As hair stops growing, the shafts eventually break, leading to male pattern baldness. After menopause, a woman's production of estrogen no longer counteracts DHT, leading to female pattern baldness, a condition in which the front hairline is preserved, but there is a general thinning of the hair.
According to HowStuffWorks, Alopecia areata is an autoimmune disease that attacks the hair follicles. The condition causes the hair to fall out and growth to slow. Under normal circumstances, hair begins to grow again after a few months. A related condition, alopecia universalis, causes loss of all the scalp and body hair.
Scars often stop hair growth in the affected area. Burns, skin infections or injury or X-ray treatments can leave scars on the scalp, eliminating hair follicles so that hair no longer grows in the area.
Telogen effluvium is a condition affecting the resting phase of hair growth, which is the final phase. Under normal circumstances, an individual loses 50 to 100 hairs daily in the hair growth cycle. However, those suffering telogen effluvium lose more hairs daily. Most cases last about six months, but chronic versions persist for years. In some cases, the hair begins to grow again on its own, but in others, medication helps to restore the growth cycle.