Well, this is a difficult question to answer because you haven't explained the situation, why she left you, and whether or not you want her back. If you want to move on, the best thing to do would be to end all communication with her and keep yourself occupied with things. Go out, hangout with your friends, find some new hobbies, etc. The more you do this the less you will think about her and you will eventually move on.
However, if you want her back, it's a bit risky because you can't make someone else want you back. It's 50/50 chance. The worst thing to do would be to call/text/email/facebook chat her constantly because all that will do is push her away and make you look desperate. Your best bet is to NOT contact her whatsoever. If you are still talking to her, STOP immediately. Don't even ask her friends about you. Have you ever heard that absence makes the heart grow fonder? This is definitely true especially if you end contact recently after the breakup. If you do this, and act like the breakup doesn't bother you, she will wonder where you've gone, what you're up too, and MAYBE think twice about whether or not breaking up with you was the smart choice. I would not contact her for about a month. Even if she texts and calls you before a month has past, DO NOT answer. This will definitely worry her and make her wonder if you've completely moved on. At the end of the month, just give her a buzz and say something like "hey, how's it going" you know like small talk. If things lead towards rekindling your relationship then great, but if not, then honestly the best thing to do would be to move on. I know it's hard and easier said then done, but you have to or else you'll never be happy. If it's not meant to be, then it's not meant to be and you'll eventually find the right girl for you.