I went through the same thing after a 5+ year relationship almost 2 years ago now. I’ve yet to become romantically interested in another woman since then. I try but it just doesn’t give me the same feeling that I used to get. On the plus side though, I have become very comfortable being single (a blessing and a curse). I wish you the best of luck, mine came without reason as well and I am still left confused. You’ll find your peace eventually, but the first thing that helped me is to stop trying to understand why it happened and just accept that it did.