Accused of not citing a source in an english final paper So I turned in my final paper for English and was soon met with this 

Dear -----,
On April 28, 2018, you were involved in a possible violation of the Code of Student Conduct.
Specifically, it is alleged that you failed to cite an online source used when completing your ENG ---- final paper.

I am a first-time freshman at this university and have absolutely no idea what to do or expect. I've combed through my paper multiple times but there is nothing present that indicates a misses citation. I have citied everything I have used and have included works cited page at the end. Is there any way for me to fight this and prove myself not guilty?
It sounds like you're confident you didn't do anything wrong, at least not on purpose. So I wouldn't panic. 

The instructor may have used an automatic plagiarism detector. Depending how it was configured, it may have been too strict in identifying similar text, and the instructor didn't apply human judgement. You may have accidentally and innocently written a sentence that is similar to something online. In that case, I would just explain what happened.

Alternatively, someone else may have copied from you.