It feels like my child is getting alienated. How do I get my child back?

Parenting. A good place to start is just talking to your child about your concerns. However, counseling would be a good idea, too. .
Get down to earth. Before you can correct anything you should get a feel for what the alienation is all about and any potential trriggers. To do that, try and connect with your child by spending quality time together and asking non threatning questions. Depending upon age, look for signs of teasing, bullying, drug use, sexual id conflicts, etc and then get into their world and help them out as best as you can.