"Are these signs that the marriage has broken down"
Yes. As you indicated, it's not the kind of married life you want to have. Lack of physical and emotional intimacy can indeed cause strain in a relationship.
There may be different reasons that affect her behavior such as: hormonal changes, complacency within the marriage, possible infidelity on her part, depressed mood, etc.
The statement that she had made to you that you ought to look elsewhere for sexual intimacy is a red flag.
If she is willing to talk to you about what is affecting her behavior/feelings towards you during the last year or so, it is something that has to be talked about honestly. Try to find out what she's experiencing, expecting of you, if she is blaming you for something, if her mood/libido had changed and hence affecting her desire, etc.
This is something that has to be dealt with on both ends. You can't fix how she feels if she is reluctant to speak to you and address what has been going on with her for the last year. Let her know that you care about her and want to understand what is taking place that is changing your wife into a stranger. Ensure her that you want to work on having a good marriage for both of you, and all you're asking her to do is reciprocate in this endeavor.