Damage to the stem at the point where it leaves the main stem, die back because of some other, more serious problem which may or may not spread, drought if its a young plant, hard to say really without lots more info and sight of the area/problem.

Yes I have to agree with bamboo, if it has been knocked or wind damaged that bit will die. What does the rest of the plant look like? I thought mine would have suffered this winter but it looks as if it has more blossom than ever.