School work is a challenge, and successful pursuits in school are often a precursor to successful endeavors in other areas of life. That's why a bad grade is more than just a reflection on your performance; it can also be perceived as an indication of your true character.
Sometimes, in spite of your best efforts to grasp course materials, the results of your test scores or your term papers are less than satisfactory. Sometimes, let's face it, you may not have put your best foot forward in your studies, and your grades reflect that lack of effort. Regardless of how you arrived at the destination, at some point you may find yourself coming face to face with a bad grade, and your remorse, while too little and too late, is real and you want to find a way to redeem yourself.
There are a couple of ways to go about this with some hope of a positive outcome. The first approach is to attack the problem with all the integrity you can muster and schedule a private appointment with the instructor outside of the chaos of the classroom arena. With the undivided attention of your instructor, explain the areas where you felt you did not measure up or confess to a lackadaisical, misguided method of preparing for the test or assignment and promise there will never be a repeat performance. You may touch the heartstrings of your kind-hearted instructor enough to earn a new opportunity to show what you know. If you are this lucky, don't blow it the second time. Prepare for your next exercise with all the tenacity you can rally.
In the very likely event your instructor is sympathetic but unwilling to bend the rules on your behalf, you may have to own up to the fact that you made a mistake. One bad grade is not likely to destroy the work of an entire semester and certainly won't be enough to derail an entire educational experience. Accept the fact that you have put a smudge on your record and work with great diligence on all future tests and assignments to achieve top grades to help average out the dismal results of one grade.
Above all, don't be too hard on yourself. Nearly everyone produces a performance that is less than what they had hoped for at one time or another. The key is to learn from the disappointment and strive to keep it from happening again.