In relationships, there will always be ups and downs. So, imo it`s just a phase. A phase where your feelings changed over the course of a relationship. Look within yourself to find out why you lost feelings in the first place and reflect on them. There`s always a way if you want to gain those feelings back. Communication is the key, of course. Talk about how you feel to them, and perhaps you can solve it ther. Remember all those good things that you spent with them, how much the person had made impact on you. Maybe with that you might just find your feelings for them is even stronger in the end.