My left upper molar is broken. There is a big hole. It happened about 3 years ago. I didn't worry about it then. I felt sensitivity for months. It also bled sometimes. Now I don't feel anything. It doesn't pain or bleed. But I see that its color turned to black. Half of the tooth is black now. I still don't feel any pain. Is it rotten? I wanna know what's wrong with it and what the treatment will be.