You need to let her know, without condemning her, how difficult this time is for her. Even though she can't imagine you could have ever been a teenager too, search into your own adolescence and find ways to empathize with her based on your own teenage experiences.
If doing poorly in school is a radical change for her and she has been doing poorly based on lack of effort, then this change has to be examined. Some kids give up when they feel their best efforts will still bring them poor academic performance; some teens decide it's not "cool" to be a good student anymore because it's not a value of their peers. Many teens are just so overwhelmed with all the intense emotional, hormonal and social havoc inside them that they can't focus on academics like they used to.
I'm sure she shows anger towards you now more than she shows any other emotion. I think the worst thing you could do is to leave her alone, which is probably how she would shout you could help her. Even though she will loudly protest that she doesn't want to hear your concern about her or your suggestions about her "getting a life", you need to calmly continue to offer to "name her pain", to show her you really do understand her. You need to let her know your concerns about her difficulties but you must also let her know that you expect her best in her endeavors and her behavior. By not expecting her best (not your definition of what her best should be) you show her that you've given up on her.
Abide with her. Perhaps a couple sessions with a talented family therapist, one who sees these problems a lot, could help you formulate a game plan. I would suggest that just you and your husband go for help now. You may always offer your daughter the opportunity to have "her own person" and present the offer as a way that she could get some help from someone who is good at assisting kids her age feel happier with their lives. Let her interview therapists and choose which one she feels she could work with.