If my ex-wife moved together with my children without my knowledge, we have a joint custody, what can I do to get my rights back?

My ex-wife moved without my knowledge I have tried to contact her and my children for over a year now and I get no response. About 2 weeks ago my oldest child who is now 17 got hold of me through a friend and stated that her mother is not letting them have any contact that she went as far as changing the oldest phone number and telling the child if she finds out she is talking to me that she will locked in the house she will not have a phone she will not have anything that she will take away everything and she won't even be allowed to go to school. I have texted and called the mothers phone number for what I believe is still hers there is no reply. I have no contact with the youngest child who is 8 years old. We have a court order that stated we have joint custody the parenting time is supposed to be 50/50 (one weekday overnight stay and every other weekend.