Client not paying the pending amt.

I have a client who is not paying my pending payment from 2011. In this connection, I went personally, he said that there is no Purchase Order raised so it is not possible to pay the Amount. but we have received payment for the Invoice without purchase Order then why now they are telling this and even for the Invoice with Purchase Order, they are saying that there is no Sign on the Purchase Order. but till now we have not received even a single sign on any of the purchase order.

On the visit to their place they have insulted me by saying that you should wait for 3 – 4 years more to get the Payment. kindly tell us some solution and what are the case chargeable on the company for not paying the Pending Payment and for the Insult they made.