My client is not responding for approval of the handed over work! And keeping us waiting for his approval on work to raise the invoice.

As per the client demand we have already handed over the work in PPT format a month ago almost. He kept changing his requirements, So we served as all his requests. However, since the work seems fine, he stopped responding and asking us to wait for his team for the approval since 3 weeks now! He has deposited the fees in Escrow before starting the work, However, he has not allowed us to raise the invoice and we are still waiting for his approval and satisfaction for the work given by us!! We have reminded him to take some action or give some feedback, Yet no response. In this case, how can you help us? And what should we do?