First, take comfort. You are not alone, especially these days, in having a chapter of your career that you would like to leave out of the story. All kinds of people get into difficult work situations. But few know how to talk about them in a constructive manner. So look at your personal nightmare as an opportunity to show your communications skills and your professional judgment.
Your answer should be short and honest. You don't want to sugar-coat the truth. But you do want ...