From the perspective of the Mishna, the sacrifices that are brought at the end of the nezirut are so critical to the nezirut itself that if a person accepts nezirut and then discovers that the animal he had intended to bring as a korban had been stolen, he is not a nazir. This is only the case, however, if the theft had already taken place when he accepted nezirut. If he accepts nezirut and the animal is stolen later on, his nezirut does take effect, since this is considered nolad – a situation that developed after the vow of nezirut had taken place – and ein pot’hin be-nolad: we do not undo a vow based on an event that takes place after the vow has been taken.
The Rambam teaches that in the case where the animal had been stolen before nezirut was accepted, there is no need to even have the vow annulled by a rabbi, since it turns out to have been made in error. Thus, even in the case where the animal was stolen later, the Mishna is simply saying that the vow was valid, leaving open the possibility that the man can approach a rabbi and appeal to have the nezirut annulled on other grounds.
The Mishna points to a disturbing – and interesting – case, where the tanna Nahum the Mede erred with regard to this halakha. When the Second Temple was destroyed, nezirim who arrived in Jerusalem with the intention of bringing their sacrifices discovered that they would be unable to do so, creating a situation that they would never be able to complete their nezirut. Nahum the Mede asked them whether they would have accepted nezirut had they known that the Temple would be destroyed and that they would be unable to bring their sacrifices. Upon hearing their answer that they would not have accepted nezirut, Nahum the Mede released them from their nezirut. Upon hearing of his ruling, the Sages proclaimed that this would work only for those people who accepted nezirut after the destruction had taken place. If they had accepted nezirut while the mikdash was still standing, the situation would be considered nolad and they could not have their vows removed so easily.