What Causes Delay In Menstruation Other Than Pregnancy?

my daughter who is 25 yrs and overweight suddenly since 3mths have no periods we did sonography and all hormones test every report was normal she is very stressed in her work no time for any activities bec of her time factor she works at vashi and stays at warden road she leaves at 8 in the morning and home at night she come after 11 pm