Uh, that's not really true. There's always a reason. The reason may seem inane, but there is always a reason. (Okay, the girl could be psychotic or just fucking with you when she said she wanted to go out. But let's disregard those two options as highly unlikely for now.)
Girls themselves usually can't really explain the reason though, they'll say something like "I just don't feel attracted to him that way" because they think that's
how it works. And then guys will think the same thing: "Well, I can't really be blamed since the sparks
just weren't there." Both of my straw-people think that attraction is something that either appears or doesn't, and you just have to (metaphorically speaking) chop down enough grass to find the rupees.
However, in 99% of the cases, the guy (or girl) has behaved in such a way that
drives the girl away (or guy, for an example of this, check
this guy's story). The problem really lies in what we are taught by "society" as the norm for treating girls. They say you should be courteous to a lady. Buy her dinner, take her out and pamper her with gifts. Support her when she's down and be there for her at all times. Treat her, in short, like a princess, and she'll be yours forever.
But in fact, girls hate that shit. And they may not
realize they hate it, but they (unless they're emotionally damaged, and have an unhealthy need for support) really don't want to be treated like they're made of cotton candy. Sure, it gives them a nice boost of confidence, but it doesn't spark any attraction to you. In fact it will kill any attraction that was there.
My bet here is that you did just what I described, and this exact thing happened. It's not so strange--my first relationship was the exact same way. I think this is why most first relationships don't work out.
So what you need to remember is that you keep treating anyone you get involved with as a human being. Tease them, have fun with them... Don't put the pussy on a pedestal.
For this particular case I'd say just try to move on as soon as possible. It's almost impossible to get a girl to like you again once you wussed out around them, cause they'll remember that forever. And it's generally a good approach anyway--seriously, you can get someone else (I could), it's her loss. And the first relationship pretty much never works out, so it's not like this is a huge deal. (Like mortis says that's easier said than done, but he's really right so I want to repeat his point.)
If you do want to be friends with her it's smart to get a new girlfriend! Not only is it awesome to have a new girlfriend in general, it also says to her "I'm over you, we can have a relatively normal friendship".
Jam it back in, in the dark.