If your pavlova has cracks on it or has collapsed, it means that the center of the pavlova shrunk away from the meringue. Again, this can happen if you over whisk the egg whites. Alternatively, the other cause of this issue is baking the pavlova in too hot of an oven and/or cooling too quickly. You can turn the oven off and leave pavlova in the oven overnight to slowly cool down.