Lots of things
There are a few reasons including thyroid, pituitary gland, and ovarian problems.
If you have missed your period for some time, it is best to consult with your ob/gyn to rule out more serious causes. Secondary amenorrhea
is also common in female athletes (e.g., gymnasts, marathon runners, or triathletes) due to their loss of body fat.
Answer assisted by David Escobar, Medical Student