Back off a bit from this situation. Obviously, lectures, threats, incentives, and all your help haven't worked and have made your home an unpleasant place for everyone. Focus now on trying to discover what is causing your child to simply not care about school.
If you believe that your child feels alienated from her teachers, talk with one or more of them or the school counselor about how closer relationships can be developed. Perhaps, a teacher could take the time to speak with her more in class or work with her before or after school once a week. Is there some small group of students your daughter could join in order to work with one of her teachers on class or extracurricular activities?
Social problems for seventh graders have two sides. Not having friends and feeling unpopular can lead children to be depressed and cause them to do poorly in school. On the other hand, being too socially inclined can cause students to neglect their studies. If your child is having problems adjusting socially, the school counselor will have good advice for her. If she is going overboard socially, try having her teachers sign a form each Friday indicating whether or not your daughter has handed in all her homework. Only when she hands in all her work does she get phone or some other privilege the next week.