A common reason for girls to stop having periods is that they are underweight, or else, that they don't have enough body fat. Girls who do strenuous sports such as gymnastics, and distance running, are at especially high risk for this. The body appears to have a built in mechanism that tells it to stop having periods when the percentage of body fat drops below a certain point. In girls who don't have periods for a prolonged amount of time, the lack of estrogen prevents proper bone mineralization, and makes the bones weaker. These girls can have significant problems with osteoporosis as they get older.
For a girl who is underweight, getting back to the proper weight will often induce the periods to start again. For high level athletes who have extremely low body fat, but are not underweight, it can be a bit trickier. Sometimes, hormone replacement is necessary. If your daughter is very thin, or is a competitive athlete, she should see her doctor sooner rather than later.
Stress is felt to be responsible for some girls who skip a few months of periods. The reason for this is not totally clear, but for some girls, counselling and a simplified lifestyle can rectify the problem.
There are also some uncommon medical conditions that can cause missed periods, and these can be assessed readily by a good physical examination and a few lab tests.
As a final note of advice, most girls who miss one or two periods in the first two years are healthy and don't need any special evaluation. Once a girl gets to three months of missed periods, however, I think it is important to have her see a health care provider.