You have to contact the professor to find out if you missed turning in some kind of an assignment, missed a test or something he/she did not get. An I is better than being failed for not having all the work in. It could just be a misunderstanding as well, just contact him. He/she is the only one who can tell you.

I had that happen last semester, something that did not a prof supposed did not get that sent electronically to her so she just asked me to resend it, she would grade it and change the grade. Just call them.