Is she still talking to you and is she willing to give the marriage another chance? It does not have to be anything you have done.
If she is not willing to talk and take some responsibility for her own contribution to the relationship, there is little you can do. It takes two to sustain a marriage. Regardless of what had triggered her behavior- mid life crisis, an affair, trying to figure out who she is, etc. if she does not own her part in this situation, you cannot repair something that you don't even know what it is.
If anything, what do you think she's willing to do?