Root canal and crown on back tooth. Now it has broken off at the gum line?

It sounds as if the post has come out of the teeth . If this is the case it may be able to be recemented. There could also be additional problems but without an xray it's impossible to say. If you are concerned I suggest going to another dentist in your area. 

The outcome will be based on why the crown fractured. If there was a deep cavity underneath or part of the tooth broke it might need to be pulled or could involve more procedures such as crown lengthening to save it. Make sure you let your dentist know about the concerns you have or if there is someone else you feel more comfortable with please seek them out as well.