If a dentist puts a crown in crooked, and the black at the top shows, then It comes loose, and now the black on top doesn't show anymore, couldn't that mean it just wasn't in right and that's why it has loosened?
Was my Dental Crown Attached Incorrectly?
Doctor Answers 2
Gluing in Crowns Can be Tricky
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.