Why did he ignore me?

I have a crush on this guy in my gym.. We both smile at each other and say Hi whenever we see each other, it always seemed to me that he's interested as well.. Yesterday he ignored me without any reason.. I mean I didn't to anything, just a day before he smiled at me and yesterday I don't know what happened that he didn't look at me even once, usually he looks at me all the time

Why did he ignore me?
Post Opinion