What should I do?

Nothing had to happen for you to stop liking him. Feelings change, sometimes when you least expect them to. I would say in your case you need to think about why you like him and then think about those reasons....Do those reasons fall under a boyfriend category or a friend category? And talk to him about it....Don't let it just come out of the blue, you never know he could be feeling the same way.

It's a thing called teenage lust. You obsess about a boy unil you makeout with him, or whatever you goal was, then when it's not how you imagined it to be, you lose that feeling. Simple.

^^hit it right on the head