I'm in the process of accepting a 30 year friendship is over, OP. So my sympathies to you as i know it's a sad and confusing thing. I've been thinking of starting a thread about it myself actually.
We were friends since school and would laugh like drains together as well as mulling over the sad or mundane stuff in our lives. In particular we've talked for literally hours over the years about her counceling sessions about childhood issues. Apart from her DP i was the only one who 'knew her' that well IYKWIM.
Then 15 months ago i suffered a late MC :( I was in a bad way over it but she was useless! Really - no support at all, and it's since then that she doesn't answer my emails or return my calls anymore :( 
Last year after my MC i got engaged to my DP. She and her DP were invited to the wedding of course. I hoped in fact that would be the catalyst for things to improve between us. Something happy. I had to ring her to get a reply to the invite as she hadn't RSVP'd after 4 weeks. At the time i asked if there was anything wrong and she said no, not at all.
At my wedding (3 months ago) everything was fine. Just like old times. We hugged and laughed and chatted. Since then - zilch again. I sent a thank you note for the prezzie. A few weeks ago I sent her a couple of copies of the proffesional wedding pics - a lovely one of her and her DP, me and my DH. with a short note with general news, asking how she is etc. No reply or contact from her at all :(
I know she's 'ok' physically because a family member lives near her and sees her out and about.
So i've given up sadly. I don't know what to do. I'm particulaly irked at the time and effort i've spent in the past listening to her going over and over all her issues and when i had a crisis she was absent. Not much help to you OP i know.
Gosh sorry this turned into such a ramble
i think i needed to let it out.