Claim of amount from Government Hospital in Kerala

 We have supplied stents to government hospital in Kerala. They were earlier regularly releasing our outstanding payments, but for the past one year they havent released any payments and saying that they have no funds. We have got tired writing them to again after again and getting no response from their side. How can I claim my money back which is due over approximate 3 years from government hospital and what steps should I take ? Please advise.