Friendships

My mum did this to her neighbours of 50+ yrs. When dad died nearly 30 years ago the husband took on the handyman jobs for mum and the wife would call in once a week for tea and natter as well as the long term hour long chats over the garden fence when they saw each other. Then suddenly mum stopped going in the garden and wouldn't answer the door to them.
The sad reason was mum had dementia and she knew at that early stage that she couldn't hold a conversation anymore. So she chose to ignore them.
Fortunately having grown up with their daughter I was able to explain. The three of them came to mum's funeral.

Been there, done that, wear the T-shirt.
In our case it was gradual, not sudden and the couple in question went "up in the world" financially. They moved to a lovely house and started friendships with their affluent neighbours. I still see the wife a couple of times a year when others are there but haven't seen her H for about 5 years. Fortunately my H has forgotten him so never queries why we don't see them anymore.
I find it sad more than any other emotion but I didn't always feel this way.
This was a friendship of many years with the C growing up together.

Depression is the likely answer, and the longer she leaves it to get back in touch the harder it is. She may not want you to see her how she is now.

You were probably surplus to her requirements in some way.
My guess is that you did nothing at all.

It could be that she got depression. That can make some people change their ways a bit.