What is required of one who abandoned the performance of 'Umrah due to crowding after having entered into a state of Ihram for it?

You made a mistake by turning back from performing 'Umrah due to the crowds. You must return to Makkah to complete the 'Umrah. Indeed, whoever enters into a state of ihram for Hajj or 'Umrah is not permitted to exit it except after completing the rituals, unless they are prevented, in which case they may exit ihram and offer a sacrificial animal (hady), or if they stipulated, at the time of ihram, that they would exit ihram upon the occurrence of an excuse. If you committed any prohibitions of ihram, such as cutting hair or clipping nails, then you owe a fidyah, which is a choice between fasting for three days, feeding six needy persons, or sacrificing a sheep. If you are unable to return due to lack of funds or similar reasons, then you may exit ihram with the ruling of one who is prevented (muḥṣar).