If you have been having regular 28-35 day cycles prior to this, if you are sexually active and not on any contraception
and if you have not had a period for a month now, it is time for investigation. You should get a serum beta-hCG measurement and a trans-vaginal sonogram to exclude pregnancy
. You should see a specialist to exclude other reasons for amenorrhoea like abnormal hormonal
profile, weight disturbances, stress
, drug interactions and so on. Take care.