I'm in the same situation as you. Really liked this girl. We went out one night and ended up hooking up. I thought she really liked me, and I asked her out. Turns out she isn't looking for a relationship. I've been upset about this for the past week.
How do you get over it? You have to let it pass. I'm still upset about it. Hell, I'm pissed off at her because she played me pretty hard but I still can't stop thinking of her. Emotions like that aren't just something you can forget. I've talked about this with her openly and it didn't really help.
Go see your family, hang out with your friends for a bit, try to find another girl. That's what has helped me feel better.