Regarding service tax issue

Hello Sir, I was providing man power service to a company under labor contract. Whatever I recieved as salary for the employees has been paid to them. No service charge were collected from the company on the salary. The period of contract was 2011-15 I have filed IT returns showing all the money recieved are paid to the labours. I have not charged or received any service tax on the salary of the labours. I got show cause notice from central excise charging the service tax on the entire sum that I recieved from the company and also the penalty. when I approached to the company.. the company director's says that they used to pay the service tax on the gross amount of the whole project..but when I said these to the service tax officer..he denies my words.. I am now completely abondaned. Is there any chance to bill the company for the service tax which have not charged them earlier? Please help me with your expert knowledge.