sorry to hear this. I know its such a shock. I found it hard not to feel really guilty and really worried, but for her its important to try and stay calm and consistent and try to understand how bad she is feeling, and how best to help. Not sure how old she is, Its best to try to push for a camhs assessment via the GP , as if things do get more difficult, they are aware of her. Some schools can be great, but often counsellors are not very experienced.
Keep an eye on her sleep, socialising, marks at school. Sometimes, when young people are first found out, they can be more vulnerable. Make sure she knows you love her, however she is feeling, she is not letting you down and together you will help her find support and help to find alternative ways of coping. Don't tell her to stop, before she understands other strategies, but do give her plasters and sterile wipes etc . Ask her if there are things she likes to do, which help her relax and keep calm, music, candlelit baths, my DD found it helpful to write down difficult thoughts in note books.
Is there a relative, she might open up to, someone a bit closer to her own age, who might be able to talk to her as sometimes its hard to admit feeling so bad to mum, and my DD didn't want to burden me with her problems.
Look at Young Minds website for yourself and show her some pages , assure her that lots of other young people struggle at this age. Help her understand that if she is feeling depressed, it is an illness which can be treated. She may not understand why she is doing it as it may be a combination of factors, so if she cant tell you yet, just let her know you are there to talk.