Plan B should be taken within 72 hours of unportected sex. The first one should be taken as soon as you get them and the second one should be taken 12 hours after the first dose. If you take them correctly there an 89% chance of preventing pregnancy. They work better if both pills are taken within the first 24 hours. If you didn't take the second one, you could still possibly get pregnant. The pills will not work if your already pregnant and won't affect an exsisting pregnancy.
Hope this helped Good luck to you.