Getting it to regrow skeleton is tricky but can be done if your parameters match mine, and if you can keep the polyp in a shallow glass bowl to protect it from drifting in current.
Sometimes, too, old coral skeleton will hide life in its depth and regrow as conditions improve.
Suggest you imitate those parameters asap, as there may be more bailout: it's a sign of water problems or incorrect lighting, usually too little [too much just tends to bleach a coral.]