Hi,
I have personal experience of this. I have struggled with self harm since I was 11 and my daughter self harmed at age 10.
It felt horrendous and like I failed as a mum. I questioned what I'd done so wrong, if it was my fault for her growing up with my scars etc.
It's ok to cry, to worry, to feel all kinds of things. I felt like it was something I had to immediately fix/action/stop, like I had to solve it. But honestly, give yourself and daughter time. Allow yourself to think, allow your daughter to come to terms with telling you and how that feels and how much she feels like explaining, she may not even be able to.
I echo the pp who said check her phone, however try not to do it in a way that makes her think she's done anything wrong, because it's likely she will see it as a punishment and feel shame and guilt. Ask about friendships, if she talks to any friends about how she feels and if the friendships are positive and helpful. Even if she doesn't share anything she might think about it. There's a very real issue of kids creating a whatsapp etc group and detailing self harm, psychiatric hospitals, not eating, etc.
It might be that she opens up little by little, and I know the feeling of wanting to know everything right now. But it's not likely.
You've got this, you obviously care a lot and that's what she'll need, so you're already there