Life insurance claim rejected - non-disclosure of pre-existing disease

My husband passed away 2 months ago due to road accident. He had a term insurance policy of 50 lakhs crores from HDFC Life which was bought 6 years ago. When we filed the death claim, the insurance company rejected it saying non-disclosure of pre-existing disease. We have submitted all documents including death certificate, policy papers, hospital records, and FIR. The insurance company is not giving a proper explanation. We are a middle-class family and this claim amount is very important for us. What can we do now? Can we file a complaint with IRDAI? Should we hire a lawyer? Has anyone successfully challenged a claim rejection? Please guide us.