Very gently say that they have the symptoms and it is a very common symptom not to want to face or deal with the illness but it is an illness and as her mother you have a duty of care towards her and want her to co-operate with getting better.
I believe in loads of easy, tiny interventions. Start with diet. Cut out sugar, add vitamin supplements (especially Vit B complex, Vit D spray, magnesium and herbal iron supplement) and lots of mood-raising foods: nuts, turkey, dark chocolate, salmon, bananas etc.
Exercise is vital - drag her out for a walk in sunlight every day, or get her to do a yoga with Adrienne with you.
Buy some CBT for teens workbooks and do a page or two a day. She will resist. Point out she doesn't have to like the work or believe it makes a difference for it to be worth doing.
Make sure she bathes or showers at least every other day. Help her keep her room clean and aired and tidy.
Line up lots of feelgood stuff to watch together on TV - comedy shows, upbeat music and nature videos, TED talks etc.
Help her plan a small thing each day to look forward to, a bigger thing each week, even bigger each month and one major thing a year.
Ask her to trial doing affirmations, meditation etc and to keep a journal about which of all these interventions helps her in any way. Gratitude journalling works if done the right way. Not 'I should feel fine because I have XYZ, but I feel shit but at least I also have XYZ. Get her also torecord all her achievements, however small, to challenge any black and white thinking.
None of these are cures but they create cracks in the surface of the depression.