Hi Julia from New Zealand. It's great to see so many responses empathically attuned to your situation. I cannot add anything to their comments - except to say, "Well expressed, people!"
On the prospect of any communication with this woman who wordlessly unfriended you, I would not dignify her loveless act with any communication whatsoever. If she can do that to you after 20 years, I'd say there is probably no way you could reach her; your well meant words would be trashed. She may even have the satisfaction of "seeing" your hurt in the words you would write and conclude you deserve to feel that way. She would not be the first witness to luxuriate in acts of judgemental unkindness. Don't give this woman the satisfaction - ever!
I like what an ealier poster said: She likely was never a "true" friend at all. To cut you lose so swiftly reflects ugly organization-mindedness.
Your hurt reflects your genuine thoughts that it had been a genuine friendship - to realize it wasn't cannot help but hurt.
You'll move on from this in time - but move on from it, you will. Here's a splendid opportunity to nurture the friends you still have and to develop true friendships not based on goose-stepping organizational belief systems. Best