First, let me say, I'm sorry for your loss.
Second, we're having similar problems with one of our sons who's a little younger than your daughter.
What to do depends a lot on what else is going on? Is it possible that her attitude about school, food, and anything else is related to the death of your husband, presumably her father? Has she lost interest in other things that she used to like? Has she been getting involved in "risky" behaviors or has her personality changed in any other way? If so, she may be grieving or experiencing depression, or something else. Depending on your relationship with her (both now and in the past) is it something you can talk with her about? If not, is there someone else (pastor, friend of the family, counselor) that she could talk with? Family counseling might be in order.
If it's not depression, it'll be up to her. I would set very clear rules for your expectations in her grades, if you haven't already. Make sure she understands them (not necessarily agrees with them). If she's truly not understanding something, there may be teachers at school, or even classmates who would be willing to meet her after school to help her out.
If she's choosing to ignore her grades, she's on her own. Not only should she get the grades she deserves, but she should receive a consequence at home. Grounded to the house until she brings grades up, no more after school activities, etc. She should also understand, that if her grades are poor enough, you would support her being left back in 10th grade instead of moving on to 11th grade with her friends.
I know it seems rough, but she's getting closer to being an adult. It's hard to watch them do things that can make their entire life harder, but if you ride her back constantly about grades, watch over her doing her homework, or pay for tutors when it's a motivation issue, she'll never learn to take responsibility as an adult. It will hurt her ability to keep a job and get promotions and better pay. She'll be an adult, but still dependent on you or someone else to keep her on track.
Best wishes.