What are the causes of secondary amenorrhea?

A 25-year-old unmarried girl comes to the endocrinology OPD with history of amenorrhea for past 7 months. Her menarche happened at the age of 13 years. She has no other complaints except for mild lethargy or fatigue. She has no known endocrinological problem. She complains of some occasional episodes of sweating and palpitations. She has no complaints of headache or any visual disturbances. She has no complaints of vomiting or any change in weight. She does not give any history of any drug intake. She has not taken any treatment for amenorrhea till now. She has not undergone any gynecological procedures till now. Prior to seven months, her menses were regular. Her mother is concerned about her amenorrhea as she is getting married next month and her future conception prospects.  Her height is 160 cm and her predicted height is 158 cm. Her weight is 60 kg. She has normal secondary sexual characters development. She has no features of virilization. She has no similar history in the family and she has no hirsutism or acne. She has no galactorrhea.

What are the causes of secondary amenorrhea?