When family law court has ordered parental visitation and the custodial parent is violating the court ordered visitation by preventing access to the child, you must petition or motion the court for enforcement of its own order. 

If you cannot afford a lawyer, you should simply contact the judge's office who ordered the visitation and ask the judicial assistant to set a hearing at which you can move the court to enforce its original order.

This is not legal advice and should not be taken as anything more than general information.  In no way, should this general information be considered as legal advice.   You should always consult an attorney licensed in your state to discuss your issue in depth.