many reasons for a late/irregular period. STDs, hormonal imbalance, birth control, antibiotics, stress and change in diet. its a long list u need to contact ur dr for some blood work to make sure everything is ok
good luck!
Many additional questions to ask here but I will try to answer with what you have given me.
Without knowing when your last period was, if you are on bc, age, or if you are sexually active, or any more information, there are many things that can cause a delay in menses.
If you are 100% positive you are not pregnant, there is typically something that has occurred to cause the interruption of ovulation. Normally it is a stressful situation has occurred in the recent past. Additionally, if you are very thin and extremely active it is not uncommon for women to have lack of menstruation off and on due to lack of fat cells, since our hormones are stored in our fat cells.
This is a very limited response since I have so little info to go on.