I've often had texts never arrive to my sister and we've sat side by side checking the conversation and found quite a bit missing.
So....don't think about it anymore.
It was a phone glitch. ...you realise now. You extended the olive branch. But...going over who is right...really isn't helpful. It's just continuing and building on the thought you had that "she doesn't care as didn't respond". It won't help you stop feeling down. But what might help is picking up the phone and calling your friend and say "I'm down...could you come over and distract me...or lets go for coffee" Moving forward, dealing with your flat mood and preserving your friendship is where your energy should be focused.
We all now know...life is far too short to ruin friendships over a missed text.