I know the feeling, it's happened to me before! And if you try to explain to the teacher, they automatically think the person is lying. I just think it's best if you talk to your parents about this and try to get them to talk to the teacher via email or on the phone so they can work something out. If nothing good comes out, then you could ask your teacher if she could allow you to do any extra credit work or an additional project which would push your grade back up.