The morning after pill works by preventing, or delaying, ovulation. This means that when the sperm enter the fallopian tubes, there is no egg there to fertilise so no pregnancy can occur. By taking emergency contraception as soon as possible after unprotected sex you reduce your chances of ovulating. If ovulation takes place between unprotected sex and taking the morning after pill, the pill will be ineffective in preventing pregnancy.
If you have taken emergency contraception but your next period doesn’t arrive, or is over 7 days late, then you should take a pregnancy test.