How can i have my child brought back to the united states?

last year i gave my girlfriend permission to take my 9month old daughter to belize,we went to get a notarize letter to attest they will be back in three months. now she refuses to bring my daughter back. my daughter is an american citizen and so am i. my question is what rights do i have and how can i go about in bringing my daughter back legally back to the United States?