Snapchat is usually not considered a serious form of communication and just for fun (at least that's what people in my generation think). So the fact that she doesn't text you back is not only annoying but it's also a sign that she may not take you seriously as a friend anymore. Of course, she could just be a bad texter but that's highly unlikely if she is using her snapchat which means she has her phone with her often.
I don't know how old you are, but I can tell you from my experience that good friends I've had in fourth grade are either no longer my friends or we're just regular friends. Yes, it's been over a decade since fourth grade for me but I had started to drift from them much earlier than that so it's actually pretty natural for that to happen. The time after fourth grade is middle school (and fifth grade) and kids tend to change a lot during those years. From personal experience and observation, middle school is a time where people lose a lot of their old friends and start to gain new ones.
Also the fact that you are scared to talk to her should be a sign that she is no longer your best friend. A best friend is someone who you can open up to without fear. If you are worried about what she'll think if you contact her, then you are already subconsciously aware that you've drifted apart and that your friendship is no longer the way it was before.
But if you want to talk to her, then you can start out with something like "hey, I haven't heard from you in a while. I was just wondering how you've been doing!" If she doesn't respond to this, then that's her loss because you've said nothing wrong and it wasn't something that isn't warranted for an answer. Don't continue sending her a bunch of messages if she doesn't respond to that one because it will only make you seem desperate and it may annoy her which will bring the opposite result to what you want. I know it sucks to lose friends but it's sadly a part of life. I constantly lose touch with friends and even though it's unfortunate, there's nothing I can do if they decide not to get back to me. There will always be people out there who will love to be your friend and are worth your time, so don't waste it on someone who doesn't want to put in any effort.
