How do I go about getting this girl back?

So I've been talking to this girl for about half of a semester, but we've known each other for about a year. The other night I took her out to look at these cool spots in the town where we go to school, we look at the stars (sappy I know), we hold hands and stuff. For a couple days we are really hittin' it off, then out of no where she shuts down this wall between us, and barely responds to me about anything. So yea i just don't understand what happened??