I submitted my final research paper 3 days after it's due and I got a zero. My grade dropped from A+ to B. Is this fear? Is it my fault?

I took the class in summer. I have to admit that the professor announced that no late work will be accepted due to the short time (5 weeks). All the work was done to the last day of the semester. I submitted all the work except for the final paper which worth 500/3700. I got A in all the assignments and I noticed... show more I took the class in summer. I have to admit that the professor announced that no late work will be accepted due to the short time (5 weeks). All the work was done to the last day of the semester. I submitted all the work except for the final paper which worth 500/3700. I got A in all the assignments and I noticed that that he didn't grade the final paper. I sent him an email and he said that I'm 3 days late and he simply ignored my emails. I kept sending him emails and he simply ignored me. I got a zero for the paper and my grade dropped from A+ to B. What should I do? Is it my fault or his fault that I got a zero for that assignment? I think he is too harsh?
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