I don't know if it helps to know this, but I'm in the same situation. My wife quite suddenly wants to separate after 8 years of what I thought was a very healthy marriage. There were no fights or anything like that; she is just feeling overwhelmed by stress. Ironically, her decision to do this is introducing an entirely new level of stress for both of us. She keeps saying she needs 'time and space' and has recently started seeing a therapist, so what I have been trying to do is what I have always tried to do: let her know that she is loved and supported, even if she can't return that love right now. This is particularly hard to do when my own head is constantly barraged by worry and anxiety (along with my own issues with depression), most of the time to the point that I find it difficult to function, but at least I can say that my approach to the situation is consistent with the feelings I have always expressed for her.
What I guess you need to decide is whether contacting her would be a threat to your own mental health, as having a message go unanswered can start a whole new cycle of anxiety if you have expectations. It probably also depends on what you want to say and why you want to say it. Whatever force was powerful enough for her to cut off someone who loves her is not going to go away easily, so any attempts to pursuade her to come back will likely backfire. However, at least in my case, the act of simply letting my wife know that I love and support her and encouraging her to get help has at least helped me to feel like I have done what I can. I'm not saying you should do this, as I don't know the situation, but I feel it has been the right approach for me.
It might not be a bad idea for you to take advantage of the assistance that is available for mental health (starting with your GP) and do some therapy yourself. You have no control over her decisions, but you do have control over your own well-being. You can't be there for her (or your next relationship) if this situation takes too much of a toll on your self-esteem, so make sure that you are kind and caring with yourself.
