A Friend Problem

I have a problem with a friend. She's been my friend since college, so some 18 years now. Our firstborns are close in age and when the kids were little we were together ALL the time.In the past couple of years she has started working full time and now has two teenage boys and a 9 year old, so we've lost some of our commonality. We were sort of coasting along, talking weekly or twice a week until the fall when she changed, drastically. She began having marital problems, lost a BUNCH of weight, and just got altogether bitchy. We talked less frequently, her choice, not mine, as she was always busy.
I saw her in December. She came over for lunch one day. We had a good chat, but as she left I was so worried about her, her marriage, etc. I began calling more frequently. She seemed to be doing better. We made plans for an annual birthday lunch in January. She bailed on me at the last minute. We made plans again in February. I talked to her two days before the date. She said "I really need to go to lunch with you". She never called. At this point I was just pissed off. I didn't want to talk to her for a while, so I didn't answer the phone when she called. She tried twice in the weeks after the second missed date. It's been 3 weeks since the last time she called. I sort of want to call her because I miss her. We've been good friends for a long time. I just don't know what to say to her now. How do I explain the lack of talking to her? Do I try? Do I just act like everything is the same? I don't know how to do this. I don't do the silent treatment thing. I'm not good at it. I don't know how I've lasted this long without calling her. Any thoughts??