I have walked this path before. I tried all kinds of rewards and punishments to no avail. There might be a temporary improvement, but that's it.
Then I found a wonderful book. The title is "Bright Minds, Poor Grades." Please read this book. It's about making your child's grades HER responsibility, not yours. It's an easy read and will encourage you. Another great book is love and logic.
In my opinion, as well meaning parents, we have tried to stop our children from suffering consequences of their behavior and choices. It's hard sometimes to let your kids suffer the natural consequences of their choices, but it's also necessary to help them understand that's not the path to take.
My niece in MS did the same thing your daughter's doing in 7th grade. My sister in law told her I guess you'll repeat if you don't get your act together. I thought at the time how horrible, I can't believe she's not helping her any more than that. Sure enough, she failed. Guess what, the next year she was on A/B honor roll and has been ever since. She graduated high school and was enrolled in college courses while in high school. It took her failing one year to straighten herself up. I decided my sister in law was a lot smarter than I gave her credit (and also 20 years older and had raised 2 boys). :)
Another possibility is to go to school with her. I know some parents who have done this with GREAT success. Tell her if she can't manage to bring her school work home and get it done well you'll have to come to school with her to help her stay organized. I personally know one Mom who did this with her son on a skipping problem and it ceased immediately. She dropped her son off, let him go into his first class and then showed up. She was beside him every moment of his day. From then on if there was any problems at school she'd just ask him if he needed her to come and amazingly he would straighten up :)
I feel for you and know this is a hard time right now. You are a good parent and this is NOT a reflection on you. You've raised her to make good decisions and you're not through raising her yet. She obviously knows what it takes to be a good student because she's been one before. Just know you're not alone.
Blessings!
L.