Why does it hurt so bad after having my crown reattached?

My crown came off a back molar and now I have pain after they reattached it. I had no pain prior. They repacked and cleaned the tooth they said. They numbed it so I wouldn't feel anything as it wears off I'm in severe pain. I feel it deep in my bone. Please tell me I don't need a root canal. Why does it hurt so bad after having it reattached? It was only off for a couple of hours ..