I have been accused of plagiarism by my university which i have not done what shall I do to defend myself?

I am a law student and The separation of Powers is a wide ranging subject. I do a lot of wide reading and on an ignment I used my knowledge in this subject. Now they think I have cheated as my worked resembles to sources on the internet. Most law in this area is also factual like the the judiciary comprises of judges I can't change that. How is that cheating please help I am really worried?