I didn't realise it was called ghosting. If you've known her for 7 years and talked every day, then it probably took a lot of balls for her to start cutting you out. There is either something obvious that you've done to upset her (are you sure you can't think of anything?), or if not, its likely that there are little things that have been irritating her for a while, and she's made a decision to end the friendship.
It could be something as simple as - different approaches to organising get-togethers (different socialising expectations), or perhaps she doesn't feel supported some way.
Whatever it is, if it's not something obvious, I would not waste brain space trying to work it out. It's probably not an issue of 'right' or 'wrong', but just that you have some differences, or are moving in differnt directions in your life. Friendships can often drift apart.
I'd send a message saying something along the lines of - 'Hope you are OK, not heard from you for a while. I've been feeling like you've been distant from me. Not sure if I've done something to upset you; I hope not. If you want to get together to talk, I'd welcome that'. Then just leave it alone.
If she gets in touch then she obviously feels it's something specific and fixable, but if she percieves that there are unfixable differences between you, she may not respond.
Get on with your life and try not to dwell on it. Make an effort to make new friends. If she gets in touch and you work it out, that's great. But if not, try to accept you've drifted apart.