Many instructors of online classes use a program other than google to research for plagiarism. If it came up as plagiarized it could be from a wide assortment of sources. You need to have him send you proof of plagiarism first. The program they use should highlight the "plagiarized" portion of your response and a reference to where it was taken from. If your instructor is using the google method you can fight that. If he is using a program it is harder to fight it but not impossible. It could also be that your instructor is very strict. Plagiarism covers a wide array of "copying". Many schools
don't even allow for self plagiarism. It could be that your response was a paraphrase of material from the video and not referenced. It could be that your lab responses were also paraphrasing and not referenced. I give those examples because they are the easiest to do on accident. Not all instructors will flag you for this, but there are those that do. Generally, an instructor understand you read the book, powerpoint, lecture, whatever, and worded your answer from that and don't care about references. But since your response is not an original idea, and you had to have learned it from somewhere, if you don't list references, then they can ding you for plagiarizing if they are more strict. I have only had 1 instructor (of the 6 courses I have taken online) who was that picky, and luckily I was one of the few that always uses citations and references in my work. Technically a campus can drop you from the course and put you on academic probation for plagiarizing, so only giving you a 0 is a good sign. It may have been something minor and having the professor review what caused it can fix the issue in the future. EIther that, or he is a jerk for assuming plagiarism when you didn't.
I take a lot f online courses and always cover my tush by referencing. If I feel my response was closely worded to the book or other source, I reference it. I have never had an issue with doing this.
Sorry you got a bad mark for it ad hopefully you can get a better explanation from the instructor.