Let her tell you what her plans are (find out if she is just confused about the relationship or if she's been thinking about this for a while)
If she sounds confused, couple's therapy would be appropriate (but you're for the time being away) So, encourage her to do some individual counseling on her own to figure out what is going on with her. If this seems too all of a sudden for you, then, she may have made this decision recently. Try to find out if there is anyone she's possibly seeing (if she tells you)
If you would like to work on the marriage then let her know. Try to find out as much as possible as to why she had made this decision and what would take it for her to try and work the issues out.