Was my Dental Crown Attached Incorrectly?

This is a difficult question to answer because I don't have enough information.  Some possible answers:

Crowns that are made with metal underneath can sometimes show black at the gum line. 

Crowns that are made with Porcelain need to be 'bonded' to the tooth and a gap could occur leaving a black line however, if it is bonded, it should not fall off. 

Is it possible that the dentist cemented the crown with a temporary glue for some reason and it just fell off? 

Go back to your dentist and talk with them.  I am sure they will explain what happened and should help you solve the problem.  Good luck. 

I believe what has happened is that the dark part was filled with the new cement used and the gap is not filled with the white material. Slim chance that the crown was that crooked the first time..