All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If the situation is as you mentioned, that your daughter no longer believes in Allah and Islam, then this is a very serious matter; this means apostasy and Kufr (disbelief), we seek refuge in Allah from this.
The following are some guidelines which we hope will be beneficial for you to follow:
1- To be patient and calm with her; this will help you in thinking properly on how to react to her bad behavior; the outcome of patience is always good, Allah willing. For benefit about the virtue of patience, please refer to fatwa 83577.
2- To supplicate Allah to guide her and to enable her to repent. Indeed, the supplication of a parent for his/her child is answered. Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, “There are three supplications that will undoubtedly be answered (by Allah): The supplication of a victim of injustice, the supplication of a traveler, and the supplication of a parent for his child.” [Ibn Maajah] You should also observe the etiquette of supplication and choose the best times when the supplication is most likely to be answered. For more benefit, please refer to fatwas 88296 and 92363.
3- You should give her advice. Tameem ad-Daari, may Allah be pleased with him, narrated that the Prophet said, “Religion is (sincere) advice.” They said, “For whom?” He replied, “For Allah, His Book, His Messenger, and for the leaders of the Muslims and their masses.” [Muslim]
Advice does not have to necessarily come from you. Rather, it is more appropriate to seek the help of whomever you think she would listen to, and you should be gentle and soft with her and try to please her so that you would save her from her heedlessness. For more benefit, please refer to fatwa 87672.
4- You should try to discover the reasons that led her to this situation and try to address them appropriately because the environment influences the people living in it. It is confirmed in Saheeh Al-Bukhaari and Muslim in the story of the man who killed one hundred souls that the scholar advised him, saying, “Go to such-and-such land where there are people who worship Allah, and worship Allah with them, but do not return to your land as it is an evil land.” Also, the company that a person keeps influences him. Abu Hurayrah, may Allah be pleased with him, narrated that the Prophet said, “The person is upon the religion of his intimate friend; so each of you should be careful whom he befriends.” [Ahmad, Abu Daawood and At-Tirmithi]
Finally, we ask Allah, the Lord of the Mighty Throne, to bring her back to the right path and lead her to piety and righteousness and make her the comfort of your eyes.
Allah knows best.