Company did not accept resignation so i left now received legal notice

Hello sir, Good Afternoon, Hope you are doing fine. I need to talk you on some points, it would be really great if you help me. I had joined a job in 1st june 2016 but mid of june and starting of july i feel that i am not comfort with job and also inform Team leader about it. Then on 25th July i had given Resignation by saying that i am not happy with your company please allow me to leave on 24th August. But they did not accepted my resignation, later i left the job due to depression on 8th August. Immediately i got mail from management team about it. I need your help regarding this point, They are really making me frustrated what should i have to do now.