One of the common reasons for girls to stop having their periods for many months is that they have lost weight. Amenorrhea often occurs when the level of body fat in a girl goes below about 12 to 15 percent. Girls who are athletes -- particularly runners, gymnasts, and dancers -- are more prone to this as they intensify their training. It's also a common consequence in girls who have eating disorders.
There is also the possibility that there's no underlying abnormality, and that within another month your daughter will start to have menstrual cycles again. Various hormonal problems, however, can cause the menstrual cycles to stop in teenagers. Most of these problems are quite treatable, and it makes sense to try to diagnose them early.
Make an appointment for your daughter to see her physician as soon as possible to sort out the reason for her amenorrhea. The hormones that are present during the menstrual cycle are important in helping to increase and maintain bone density in teenage girls. Girls who have prolonged periods of amenorrhea do lose some bone density, and thus have an increased susceptibility to fractures now, as well as a greater susceptibility to osteoporosis and fractures later on in life.