Our sincere sympathy goes out to you parents. The grief that Christian parents of a backslidden child endure is great. This heartache is compounded when parental control is diminished or lost because the child has become an adult and is able to silence or willfully ignore and despise the testimony of the parents. In general you have three tools at your disposal:

1. You can pray. Keep your daughter in your intercessions. Ask God to act providentially to get and keep her attention and interest (even if it be through severe trials). Ask him to supply Christian witnesses to law and gospel who may be given access to her when you have been shut out. Ask God for the ability to continue to love her and have confidence regarding her return to repentance and faith, rather than growing bitter or despondent because of your disappointment.
2. You can offer clear though limited testimony to law and gospel when opportunities surface. Your daughter may not want to “talk about it” but if you promise not to engage in an ongoing dialogue every time you see her (which is perhaps what she fears most because of a guilty conscience), you can ask her for one sit-down conversation at which time she can explain to you what she feels and why and you can explain to her what you feel and why. And in that kind of setting, law as law and gospel as gospel will be your primary tools as you sow the seed of God’s Word.
3. You can continue to testify to your daughter by attitude and lifestyle that she will observe. Here I am thinking of a cheerful, happy contentment and optimistic approach to daily life that gives Christ all praise and holds no grudges to those (including children) who disappoint or embrace ideologies contrary to your own. Continue to feed and fuel your own faith-life through the gospel, and as you grow spiritually you will also grow in your ability to cope with this disappointment, trust God’s capabilities despite your limitations, grow more fervent in prayer, and ultimately serve as a billboard of Christian contentment and joy for people (including your daughter) who have temporarily at least settled for much less. The goal is to attract them winsomely to the Lord who gives his people, in addition to pardon and eternal life in glory, such blessings despite hardships during our earthly pilrimage.