I was served with a petition of custody. I'm not sure what my next step is.

I recently let my 9 year old daughter go stay for a year with her father in Tennessee. For the last 2 years, I have her her go in the summers. We had a written and notarized agreement that he would bring her home July 31, 2016. Yesterday, I was served with a petition of custody. My ex husband will not answer the phone. I haven't spoken to him since he came to Pa to pick up our child in the last week of June. I have had contact with his wife. However, they limited communication between me and my daughter to once a month. They changed their phone numbers. Call from a private number. I have sent my daughter cellphones to keep communication but we're misplaced. The last phone I sent, I received back in the mail saying recipient has refused package. I am worried that after raising my daughter for 9 years, that he could just take her away.