It caused the same in me! I took it and thought if this happens, then I'm not dead inside. There is something triggering this response. So I started doing a ton of research.
I learned quite a bit about my needs, EMOTIONAL needs. What contributed to my lack of desire, what I can do to jump start it. What I need from him to feel validated and a priority.
This is where YOU come in. What are YOU doing to make yourself a better spouse. Sometimes the men have no clue what they have done in the past to contribute to this situation.
Sometimes they think they have been just fine, and that it is just her. Many times it is because the woman doesn't know without really looking back at the marriage. Even the dating and wooing period. There are many years where a hurt could have taken place and then it gets buried, and built upon.
This may not be something YOU want to hear, but something you may just have to face. What she may be telling you may not be the truth either.
Many things have to do with desire. Bad breath, stress, how one is treated, whether one tells the other what they need or like. Whether one is even aware of what their body is doing! SO many things, it isn't a switch. It is so much more complex.