Grounds for Failure and a Mark on record?

I am a computer science major at a University. Last week, i had two hours to take a expansive test in my systems analysis and design class, and despite it being open book and open note, managed to finish it only JUST in time to get to work. However, my professor messaged me today telling me ahe believes i copied and pasted everything from an online textbook and source, I've failed the test, and i should come sign some academic plagiarism form in her office. I am a senior in college and never have i ever been accused of such, despite multiple open book tests in the past. What's the point of making it open book if you can't use the book? I can see how that's grounds to count a question wrong but failing an entire midterm and possibly a class seems harsh. I'm taking 17 hours and working 2 part time jobs. I don't need this.