The 72 hour over the counter emergency contraception pills (ECPs) can actually be used out to 120 hrs per the WHO (World Health Organization). However, they work better if you take them ASAP.
The over the counter ECPs are levonorgestrel which is a progestin. They basically tell the body that you are pregnant so why would you want to ovulate (pop out an egg). Thus, if the woman has already popped out an egg (ovulated), then it won’t work and it’s too late to prevent pregnancy. If a woman is going to ovulate in 5 days, then you have 5 days to throw this high dose progestin to prevent the body from releasing egg.
If you are already pregnant, the progestin will do NOTHING to the existing pregnancy. That is it will not cause the embryo to die nor mutate. It will not cause an abortion.