A glazed terra-cotta relief by the Renaissance sculptor Andrea della Robbia came loose overnight from its perch above a doorway at the Metropolitan Museum of Art and crashed to the stone floor below, suffering serious damage, museum officials said on Tuesday.

The fractured 15th-century sculpture, a 62-inch-by-32-inch blue-and-white lunette depicting St. Michael the archangel in a traditional pose, holding a sword and scales, was found early on Tuesday by a guard on regular rounds.

Harold Holzer, a museum spokesman, said the sculpture, which had been displayed over the doorway in the European Sculpture and Decorative Arts Galleries since 1996, might have done a flip in the air as it fell, causing it to land relatively flat on its reverse side and sparing it “catastrophic damage.”