It can be frustrating when you realised that you have forgotten to take your contraceptive pills after having unprotected sex or when your contraceptive method has failed. Normal birth pills can't help prevent pregnancy when taken after unprotected sex, thankfully, there is an emergency contraceptive. Known as the morning-after pill, it is an effective option for preventing pregnancy after unprotected sex or if your contraceptive method has failed.

Reference: http://www.mayoclinic.org/tests-procedures/morning-after-pill/basics/risks/prc-20012891/
http://www.who.int/mediacentre/factsheets/fs244/en/