All praise be to Allah the Lord of the Worlds.
Tahallul (Removal of the ritual state for Hajj and `Umrah) by shaving or shortening hair is a pillar of Haj and Umrah, and the latter rituals aren`t completed save by performing it. In addition, it doesn`t avail a person to slaughter a sheep for leaving this pillar. As for the one who didn`t make Tahallul for Hajj or Umrah after having completed all their rituals, then engaged in sexual intercourse with his wife, there are two views:
First: According to the correct opinion of the Shafie Madhab, his Umrah is nullified and he is required to make up for it. He is also required to slaughter a she-camel and distribute its meat amongst the poor people of the Meccan sanctuary, and this ruling applies to both rituals: Haj and Umrah.
An-Nawawi said: "Shafie jurists are agreed that one who had nullified his Hajj is required to offer a she-camel. As for the one who had nullified his Umrah, there are two sayings: the first, which is the more correct, is that he must offer a she-camel similar to the one who nullified Hajj. The second includes two opinions; the more correct one is that he must offer a she-camel.
The second one, according to Al-Rafei, he must offer a sheep." {Majmou` Shareh Al-Mohathab, 7/389}. Second: According to Hanafie, Maliki and Hanabali Madhab, and one opinion of the Shafie jurists, his Umrah isn`t nullified, but one who had engaged in sexual intercourse with wife-after having performed Sa`i {The devotional act of walking seven times between the hills of Safa and Marwah which are located some distance from the Ka’ba}, and before making Tahallul-is required to slaughter a sheep and distribute its meat amongst the poor people of the Meccan sanctuary, because he had violated one of the restrictions of Ihram.
Al-Merghani, a Hanafie jurist, said: "If he engaged in sexual intercourse with his wife after going around the Kaaba four times or more, he is required to offer a sheep and his Umrah is valid." {Al-Hidayah, 1/161}.
Al-Adawi said: "As for Umrah, if it was nullified prior completion of Sa`i, even for one round, then it is nullified and must be made for, in addition to offering a sheep. However, if the nullification took place before completion of Sa`i and Tahallul by shaving, then he is clear from liability, but he is required to offer a sheep." {Hashiyat Al-Adawi, 1/551}.
In conclusion, the above questioner must shorten her hair, make Tahallul once she remembers, offer a sheep and distribute its meat amongst the poor people of the Meccan sanctuary, and her Umrah is valid. And Allah knows best.