Please Help!! My Previous Employer Has Sent Me A Notice On Violation Of Post - Employment Obligation

I have previously worked for a same client which i'm working now in my new organisation and my new company doesn't have any problem on this and my client as well. but my previous employer had came to know through some source and they have sent me a notice stating that "Notice on violation of Post - Employment Obligation". And without knowing much about NDA i have signed the Document when i was relieving from the previous organisation. Now they have sent me notice that i have breached the NDA and they will proceed for legal notice within 5 days of justification.
What should i reply them? or should i ignore the notice?

You must never ignore a notice. Also without reading your agreement one cannot comment on legalities. But generally there are many things that controls limits of restraints they put on you post employment.

See this

" An agreement to restrain a servant from competing with the employer after the termination of employment may not be allowed by Court, In Brahmaputra Company Limited versus Scarth an attempt was made to restrain a servant from completing for 5 years after the period of service the court is disallowed it.

Supreme Court again laid down in Suprintendance Company of India vs Krishna Murugan, Restraint beyond the terms of service would be prima facie void and the only ground on which it could be justified is by bringing it within the scope of the exception that is by showing that it is necessary for the protection of employer’s goodwill. The court also pointed out that even if such restraint is valid it will be only apply after expiry of the term in its natural course and not when the employees is wrongful dismissed earlier."

See full here Things To Remember While Drafting A Valid Employment Contract

If i reply to the notice should i agree that i'm working with the same client and competitor?

And also i want to mention this, i have resigned my previous employer because they are firing employees and also the same project which i have worked there was almost done and they have told us the project as gone out to some other vendor. when i have started searching for job, i have got a job where i didn't know that i will be working for same client , where as my current employer and client knew about it and they have placed me in the same project again. with out knowing much about nda, i have started working on this project.Now when i ask my employer to give me support they are not ready to support me since the previous organisation didn't mention any organisation name and client says to leave and wait for another notice to reply

You have actually breached the NDA and also it is unethical. Better to accept your mistake.

Please contact some good advocate immediately who deals exclusively in service matters.
Here you have already hijacked the client of your previous company which is ethically and morally wrong.
If you have signed an NDA without reading it, it is not going to help you because it will be presumed to have been read.