You have the header for what you sent, showing the date/time you sent it, I would forward (as well as print out) the email to the right place with a simple explanation. Hopefully the professor/school will "fix" it. If your other grades were good in that subject, it won't harm much; I had to be away for a final when I had an "A" in that class and was offered an incomplete or a gentlewoman's "C" :-) I took the "C". It wasn't a subject in my major. Maybe you can ask for an incomplete and show them that you did in fact do the assignment, etc. and all they have to do is grade it. . .