What do you do if a guy asked you out but you said no even though you like him?

Rectify the problem as soon as possible. Learn to NEVER judge a book by it's cover. In other words he may not be all that physically attractive to you, but if that's all you're counting on (or perhaps trying to impress your girlfriends at how 'hot' he is or isn't) then you are going to miss out on some wonderful male companionship. Looks fade, personality stays! So either phone him and apologize (say you had a rough day) or if you are in school walk up to him and apologize and tell him you'd like to go out with him.