I'm assuming you mean what we call Viburnum davidii - this is a pretty tough plant, but may at times be affected by late cold after warm spells, or extreme shortage of water. Viburnum beetle may or may not be a risk in your part of the world, but that usually shows itself by the leaves being full of holes. If the plant has now recovered and is growing away well, then I wouldn't worry about it. If it happens again, or is still happening, a photograph would be useful if you're sure it wasn't drought.