OK,I'am 18 my brother has a friend.. this guy use to have have a HUGE crush on me that it was so obvious that everyone relised..
But I never gave him attention because I was intrested on someone else.. and so one day he decided to give me another chance and rake me out for dinner.. unfortunatly I wanted to go out but I had to travel at the same day..
He wanted to marry me.. I blew it, how can I get him back?

Let me get this straight, chronological order:

1) He likes you.
2) You weren't interested.
3) He wants to take you out for dinner.
4) He wants to marry you.
5) You reject him because you are busy.
6) He losses interest, including wanting to marry you.
7) You gain interest.

Minus all the ridiculous comments I want to make, but will refrain from doing, here are some neutral observations:

a) If he can change his feelings about you so quickly, then he doesn't really like you in the first place.

b) It's very awkward when someone wants to marry another person if they haven't even gone on a date yet.

c) You didn't blow anything, if you still wanted to go out with him now and if he still had feelings for you, you guys would be on that date now. If he's not willing to go out with you now, it means he's not as interested as you think.

Guys really hate rejection (guess girls do too) I say give him time if he was really as interested as you say, like wanting to marry you, then he may still give you a chance. If he doesn't well move on , there are plenty of fish in the sea.

Personally I don't want anything to do with someone who thinks they want to marry me without ever dating me. That is a bit too creepy for me. I would have read that warning sign and been out like the Road Runner! Beep , Beep...

Unfortunately, some people just like the chase, and once they know they can get it, it isn't fun anymore.

You didn't blow anything, his date was not your priority at the time. Actually you may have saved yourself some hard times, as nothing sets my spidey sense off quicker than someone who falls for that "I want to marry you" line before they know you, or you know them.

Too much, to fast, crash and burn. If you weren't so carried away by intense feelings, maybe you could have a more realistic view of events, words and actions.

Re evaluate your feelings in an honest realistic way.