I got arrested for something I didn t do.?

My sister lied to the cops and told them I beat her up. It was the day I got home from having my splenectomy and could barely even walk. Although I had 27 staples in my stomach and was clearly in no fighting condition the police still arrested me. I spent 30 days in jail and was charged with simple assault. I can t even begin to explain how frustrated I am having this happen to me. There has to be something I can do.