Stress , antibiotics, diet, blocked tube, no ovulation
Depending on the age, if she’s young, it should be because of the hormones, if she’s middle age, maybe because of stress, and if she’s “old” it could be because she in her way of menopause.
Stress ,any kind of hormonal imbalance,lack of proper nutrition,heavy excercise,workout,high range of temperture changes ,,or any disturbances in menstrual cycle ,stage of menarche,lactational ammenorhea,etc. May be the conditions where periods are missing or may be absent