Respond nicely, then go see him in person. If he still wants to give you a 0 after seeing him on Monday, then go to his department head. There is no way that he should give you a 0 on a test just because of some small non-class related error.

My wife got a bad score on a political science paper not because her views were not formulated and presented correctly, but because the professor disagreed with her opinion on the matter (she demonstrated how people from his native country financed a war in her native country). The professor wanted to give her an F, but she took her paper to the department head and he re-graded the entire paper for her, overriding the previous grade.