I see that you have got nit trichy in first round in ews category. So, first of all congratulations. And you are saying that you have no ews certificate. So without it you will not be allowed to participate in registration process. 

You can talk to your tahsildar or C.O. Tellthem the condition. They will make. But since there will be many applying for the same so it might take some days. But there might be an another way. You can write a letter signed by tahshildar, and then show it to the respected authorities..

Or you can talk to jee mains concerned authories. 

Thank you.