Accused of not citing a source in an english final paper
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.