Your first step is to contact your instructor directly. You will need to use email, not the Canvas Inbox. Check your school's website for your instructor's email address. (Canvas Inbox only works when you are still enrolled in the class. It won't work after the class has been concluded.)
Ask your instructor to explain to you how he or she calculated your grade. At the beginning of the semester, you should have received a syllabus with the course's grading policy. If you didn't keep a copy of the syllabus, ask your instructor to send one to you. Hopefully, at this point, there will be some clarity as what grade you should have received for the semester.
It is possible, as @chriscas pointed out, that there may be some mismatch between the unofficial grade on Canvas and your actual course grade. For example, your instructor didn't have the Canvas Gradebook set up correctly in terms of how points are calculated. Your instructor should be able to explain the discrepancy.
If you still feel that your course grade is in error, then contact the department chair or dean if you are a college student. If you are a high school student, contact your vice principal or principal.