Does God Hear Your Prayers If You Are In Mortal Sin?

Yes. We distinguish between sanctifying (habitual) grace and actual graces. Now the name ‘actual’ grace might make it sound like we’re implying other graces are not ‘real’ graces, but they are named ‘actual’ because they refer to particular and passing actions, whereas sanctifying grace refers to the state of being, the habit of holiness that persists after baptism as long as we do not sin mortally. It’s the difference between doing things, performing certain actions, and being human or, with sanctifying grace, being a child of God.

Mortal sin takes us out of the state of grace. We lose sanctifying grace and the theological virtue of divine charity, but God could still speak to us because those would be actual graces, passing actions that God can grant even to someone who is not in the state of grace. And He might grant them precisely to spur us to repentance, to Confession, and a return to sanctifying grace.

Source: Fr. Schmidt's Blog