I'm going through serious medical problem but company didn't accept my resignation request.

Hi, I'm going through a serious health problem and due to this I'm not able to speak or talk properly. I've submitted the updated medical records to the HR department where my consultant physician clearly mentioned that I need minimum 1 month of voice rest and follow the medication. I've requested my HR to give ne an early release based upon my serious medical condition but HR rejected it and instructed to buy out the notice. They continuously harraising me in this way. They have also hold my previous month's salary. This put me in a very difficult position as my fiancial condition is not good. After that I've given them a written mail about immediate resignation due to my crutial health condition and offered them to accept my previous month's hold salary as a 1 months notice buyout and request them to consider atlist another month's notice. Still the HR didn't accept it and forcing me to visit office and serve the 2 month's notice. They told me to either get back to work after receiving the fitness certificate or I need to buyout. Sadly they're not cooperate with me in this difficult situation. I'm really not able to work anymore beacuse this job was badly effected my health. I've tried everything as much as I can but the HR didn't helped me at all. I need release asap. What should I do now? Is there anything can be done from my side? I've communicated with them through mail I need help. Please.