Need Guidance From Senoirs-Its Urgent

Dear Seniors,
We need your guidance.
I am working in a consultancy and we have closed 3 positions in one of the famous retail client. We have raised invoice.The due period had been exceeded with the non-payment of the dues (For all 3 clients) beyond the date extension for the payment to be done.
Now I am not able to contact HR either by calls or by mails. And we tried higher person also through mail and there is also no response. We went directly to the client place. But they are not allowed us to meet HR. We have only one block in our mind that they are the big client and ours is newly start up consultancy. We have agreement, mail communication for closures every thing as a proof. Please let us know how to get payment from the client without making it as an issue..

Dear Vino_Peter,
Mere agreement is not enough. Did you take Work Order from your client before execution of your services?
DVD

Dear Mr. Dinesh Divakar,
Thank you for your response. I have received the recruitment details for the HR by mail and i have all mail communication from profile sending to till closure. Is it enough?

Greetings Vino_Peter,
In my opinion as pointed out by Mr.Dinesh Divekar, it is advisable to have a work order or a formal agreement with the company before you take up assignments from the said company.
There is yet another aspect you need to introspect. Whether you closed these positions earlier in the recent past?
In some instances consultancy firms close positions and within a span of few months the candidates leave the organisation. By this time the company settles the service charges. In such instances the HR department is unable to justify yet another invoice raised by the consultancy firm.An agreement regarding the duration, during which the consultancy firm will offer free replacement is advisable.
Regards