Quick answer
Determine whether the child is safe, where they are, and what the order requires; seek urgent help when necessary.
Do now
Keep the order and agreed return details available. Save messages and attempted contacts. Contact law enforcement for a missing child or immediate danger, and obtain urgent legal advice about enforcement or recovery procedures.
Watch for
Do not pursue a dangerous confrontation or cross borders to seize the child. International removals require specialized assistance; ordinary enforcement and emergency recovery use different procedures.
Official sources
NJ Courts — Representing Yourself, DC Code § 16-4602.01 — Initial child-custody jurisdiction, NY Courts — Custody and Visitation, US State Department — International Child-Abduction Prevention.