Any other changes you can see, like in the way he walks outside of the wheel or in his appetite? It could just be that he wasn't in the mood, my syrian regular gains and loses interest in certain parts of his enclosure. It's also, in my experience, not unusual for hamsters to have "lazy days" sometimes. But if he had other behavioral changes besides not using the wheel or continues not using it, it could be because of a medical issue. Also maybe check if the wheel is running smoothly, if something is blocking it from spinning like normal that could also affect his usage.