The medical term for "no period" is amenorrhea. The average age of menarche (first period) is 12.7 years in the United States. If a girl has not menstruated by 15 years, the reason should be investigated. Primary amenorrhea is a condition that a woman fail to menstruate at all. Secondary amenorrhea is a condition that a woman has menstruated but it then stopped.
By answering questions about your condition, you can find the causes.