What concerns me with your description, is that your daughter's school performance has seriously dropped off across all subjects and that this drop-off has occurred suddenly while being accompanied by an uncaring attitude, in general, about school. This severe and sudden change and her depression are indeed cause for concern. If she is moody off and on, I would view it as more "normal for the age"; if she appears unhappy often, this merits attention.
Besides making her aware of your concern for her obvious unhappiness and not blaming her or punishing her for her school performance, focus on what could be happening in her home life, school life, or personal life that could lead to her present status. Are there troubles within the family that could be making her sad, fearful, and unable to concentrate on school work? Is the schoolwork too demanding? Has she has given up at all levels because she feels she just can't measure up academically? Does she have outside activities or a job that are taking too much of her time? Has the pressure of doing much more schoolwork thrown her into feeling there just isn't enough time to do all the work? Are there any signs of drug use? Have there been recent major disappointments in her life?
If she continues to exhibit floundering and not caring at all about school and continues to be depressed all the time, not interacting with friends, staying in her room, then some professional help should be found. Make sure you describe your setting up counseling for her as an attempt to help her feel better about herself and her life, not as a punishment for "screwing up". Let her interview therapists and pick the one with whom she feels most comfortable. First ask for advice on who the best child therapists are in your area who have experience with kids your daughter's age. Good luck. You are correct in not thinking this turnabout will just go away on its own.