Modern hormonal birth control
pills are both safe (in most women) and effective if used correctly. When used exactly as you are supposed to, the birth control pill is greater than 99% effective. In the real world, the effectiveness is less because of confounding factors. The pill must be taken at about the same time every day in order to make sure there are no drops in hormone levels. Other medications including some antibiotics can interfere with the pill's effectiveness. If neither of these were factors, then it is you chance of pregnancy is low. Besides pregnancy, other possible causes of a missed period include taking the hormone pills throughout the month while missing the placebo pills. This would only occur if you lost your pills for a month and started on the next months pills at the beginning. I suggest that you start by going to the grocery store and buying a home pregnancy test. These tests, when used correctly, are very accurate and will tell you if you are pregnant. If you are pregnant, you should schedule an appointment with your OBGYN very soon. If you are not pregnant, then you should schedule an appointment with your doctor
that prescribes your birth control pills. From there you can discuss why it is you missed a period. Good luck.