Lack of periods. There are many reasons a girl can miss her period, and it would be difficult to make any list of "all" possibilities. The most common reasons are pregnancy, contraception, inadequate hormone stimulation of the uterus, low body fat, stress, medications. A less common reason can include infection causing adhesions in the uterus.
Several possabilitie. There are several possabilities. The first is obviously a pregnany.Other possible cause include hormonal imbalance, stress, the type of birth control she may be using (ie: the dept injection), athletic females, ovarian malfuction, anorexia, or just a normal varient where you can miss one month. If you miss a seond or even a third month, a visit to your gynecologist is recommend.