Goodness, this sounds familiar! A similar thing has happened to me over the past few years with an old school friend. Everything seemed fine between us, although I loved abroad so we would message every now and then and catch up when I visited the UK. We flew back especially for her wedding several years ago, and met up with her around 6 months after that, all seemed totally fine.
I ended up moving back to the UK to study for a bit, around the time she had a baby. I even posted her a gift, which she never acknowledged nor thanked me for (so I still wonder if it was even delivered).
Well, nearly 3 years ago now, we finally arranged a time to meet, and my other half and I booked a hotel in London and traveled down to meet her / her baby for lunch. She messaged to say the baby had been up all night, she couldn't make it, we can rearrange some other time. I responded and said no problem at all, I completely understand, she must be exhausted, etc, and we can do another time next month. Well, that message was left unread. And the message I sent to follow up and arrange a new date a month later also left unread.
Three years have passed and she has never unfriended me on social. She occasionally likes an Instagram post, but that is it. Messages STILL unread (so there's no chance it was an accidental muting, as surely at some point in 3 years she would have looked me up again.)
The whole thing has been seriously hurtful and made me question my entire personality and sanity. Did I do something wrong? Was I an awful friend? I've analysed every tiny detail of each interaction and our life. I'm at the point now where I will probably unfriend her soon, as I mentally gave her 3 years to reconnect, and she hasn't. It is basically a very hurtful and cowardly way to end a 20-year friendship, and particularly awful when you learn this woman workls in mental health and has 2 master's degrees in various psychology / social work / mental health related subjects!
You have my sympathy, OP. It is so hurtful and awful when this happens. Their loss, I say.