Teese, I'm glad to see that you put on your "big girl panties" and sent her an email. However, you cannot expect a response so quickly. She just had surgery and may be in a cast w/pain meds. Another thing, if her arm is in a cast, she may not be able to type right now and/or may not be home yet. There are many variables here to consider....
I'm going to give you something to think about...have you ever given any thought that maybe God had a hand in this and wants you to reach out to your MIL? Stop and think about it....each and every time one of us is focusing on the things that our spouses have or haven't done, he puts another situation in our paths to turn our focus on to the new situation. In your case, you were upset about your son receiving a Christmas gift from his grandmother, less than a month ago. The accident she had could have been worse, i.e., she could have hit her head and died or broken a hip. I know you are going to get up tight about what I'm going to suggest, but I would send a nice card and sign it from the family. After all, she is your children's grandmother.
Treese, the dynamics swirling around you and your situation will not change until someone takes the first step in making changes. I know that you have mentioned many times that you and your MIL didn't really get along, but have you given any thought that she really doesn't know how to communicate w/you, especially now with the situation the way it is? She may not know what to say to you, and yet, she's good friends w/your mother.
I was in the same situation many years ago and I reached out just before my MIL died. There were so many things that I've regretted and should have done differently the first five years of my journey and now it's too late to talk to her. I don't want to see you make the same mistakes and then it will be too late.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.