Missing a period while on birth control pills is common. Sometimes the walls of the uterus just don't build up enough to cause any bleeding. Other times, the hormone levels have simply not dropped far enough to provoke a withdrawal bleed. In any case, many women on the pill miss periods all the time.

However, a missed period could also be due to pregnancy. Many women report that the only way they found out they were pregnant was because they had no withdrawal bleed. A withdrawal bleed would most likely not occur of one were pregnant, and if any bleeding were to happen it would be very light and most likely spotting.

But don't worry, people miss their periods all the time on the pill. Remember, it's not really a "period", since ovulation and menstruation do not occur on the pill, but rather a withdrawal bleed due to the drop in hormones.
Take a pregnancy test or just wait until your next placebo week to see if you get your bleed again. If not, you might want to see a doctor.