Feeling Heartbroken :(

What does one do when a friend of almost 20 years suddenly decides that they are no longer speaking to you? I wish I knew the answer.
About a month ago, a dear friend, I'll call her Dundas, stopped responding to my messages. I have had a crazy year, living an hour away from my home, albeit with my parents, in order to be super close to work.  I know that I haven't been the best friend and acknowledge such when Dundas and I were together to celebrate my 40th birthday.  I flat out said that I was looking forward to the summer so that I could be a better wife and friend.  You see, I was only spending Wednesday, Friday and Saturday nights at home.  This meant that I tended to prefer to spend my time with my husband because we really weren't getting a lot of time together.  The summer would mean that I would have much more freedom with my time which would mean that I could spend time with some friends too.
However, when I messaged Dundas to try to make some plans, there was no reply.  I gave it a couple of days, we all get busy, and messaged again.  Still no reply.
I had noticed that some of Dundas' Facebook posts have been sort of dark and I messaged her again to tell her that I was concerned about her.  Again, no reply.  At this point, I reached out to a couple of mutual friends, one of which was kind enough to tell me that Dundas was angry with me for various reasons, most of which were long past and somewhat petty, in my opinion anyway.
I have since sent her one last message basically stating that I believe that after almost 20 years, I believe that I deserve an explanation of why she isn't speaking to me, but that too did not receive a reply.
I am heartbroken, pissed off and frustrated.  I don't know what to do.  I do not want to lose Dundas as a friend.  We always have a great time together and I miss her terribly.   My hands are tied and now all I can do is wait for her to decide to talk to me again....maybe.
One would think that at 40 years old all of this crazy crap would be over and done with but I feel like I'm back in high school and my friends are trying to choose sides between me and my ex-boyfriend...ugh!