child custody

Hi..can someone plz help me to understand how can I get my child custody back. Below mentioned is the relevant details and current situation: 1. My daughter is 2.5 yrs now. After 6months of her birth her mother expired in an accident. At that time my father in law & mother in law requested to keep my daughter as their only living hope and in good faith I agreed and they took her along. 2. My father in law works in a city in MP and are basically from west bengal. 3. After my wife's death my mother in law took my child to WB to their native place while my father in law stays in MP. For past two years my daughter is staying in a small town of WB with my mother in law. 4. Whenever I ask them to bring my child to me or to my place they deny and start fighting. And now they have sent some notice through lawer demanding life long child guardianship to my address. (I belong to a district place city in Maharashtra). 5. I want to get my child back to me. Can anyone please guide me with startup procedure, right way to revert, sections involved, place & possibility of filing the case and relevant information I should be aware of. Thanks in advance.