Plan B works by delaying ovulation, so once ovulation has already happened it has no mechanism left, and it cannot interrupt an existing pregnancy.
Levonorgestrel prevents the ovary from releasing an egg; that is its job description. After the egg is out, the pill is largely toothless. The hard part is that you usually cannot pinpoint ovulation in real time, and the fertile window runs about 5 days before ovulation plus the day itself, because sperm can wait that long. Mid-cycle unprotected sex is exactly the scenario where Plan B is weakest. If that is your situation, the options that do not depend on ovulation timing are the copper IUD (placed within 5 days, the most effective EC, and it keeps working afterward) and ulipristal acetate (ella), which can postpone ovulation even close to the moment it starts. Taking Plan B anyway is not wrong if it is what you can get, but pair it with a pregnancy test if your period is late by 3 weeks.
What to do
Common questions
Does Plan B work during ovulation?
Plan B's mechanism is the key to this answer: levonorgestrel works by preventing or delaying the release of an egg from the ovary. If you have already ovulated, there is no egg release left to delay, so Plan B has little to nothing to work on, and it will not interrupt an existing pregnancy either. The honest answer is that Plan B is unreliable if taken after ovulation. The problem is that most people cannot know their exact ovulation moment in real time: period-tracking apps estimate it, and the fertile window spans the roughly 5 days before ovulation plus ovulation day itself, because sperm survive that long waiting for the egg. So if your unprotected sex happened anywhere near the middle of your cycle, do not assume Plan B will cover you. The options that work regardless of ovulation timing are the copper IUD, placed within 5 days, which prevents fertilization and implantation and is the most effective emergency contraception, and ulipristal (ella), which can delay ovulation even as it approaches. If you take Plan B anyway because it is what you have, that is reasonable; just follow through with a pregnancy test if your period does not arrive within 3 weeks.