Use of AI

I submitted an essay a week ago and when I checked my grades I found out I got a 0 on it. I had absolutely no idea why as I followed the prompt correctly and even looked at the rubric provided. My professor left a comment and said some of my paper was generated by an AI. I have never used AI in my life, I don’t even know how to use it for an essay. I don’t know what to do and I’m upset because I worked hard on it and I got a 0 for something I didn’t even do.