It is common to panic when your significant other starts drifting away from you, or breaks up with you. Do whatever you must to force yourself not to call him, especially in the middle of the night. Do not act clingy or desperate, though that is how you may feel. Pursuing him will cause him to run farther away, and may jeapordize any chance you have of winning him back.
Giving him space shows that you are no longer hard up. This causes your ex to start wondering about you. That draws him closer. It is natural to pull away if you feel pressed, and gravitate toward someone they is not completely available.
Plan activities or attend social gatherings with his friends (but only if they are your friends, too). For example, if his friends are organizing a white water rafting weekend, then sign up. Go and have a glorious time. Word will get back to your ex that you were the life of the party, great fun to be around. This will drive your ex crazy. First, he won't want you to hanging around his friends because you might date one of them. And second, he may start to miss spending time with you if others are having such a nice time doing so.
Are you close with his family at all? Are you good friends with his mother? Then nuture those relationships on their own. Don't come across to his family and friends as a desperate ex-girlfriend trying to weasel her way back into her boyfriend's life. Honor the ties you already have with his family and close friends as stand-alone relationships, that don't necessarily involve him. For example, invite his sister to a concert (if you're friends with her) and don't even mention him during the event. Dress wild and have a fabulous time with his sister. If you break down and start whining/crying about him to his family, however, this tactic will completely backfire.
Uncover the real reason why your ex "left." It is not your fault, per se, but if you can find out where the problem lies, perhaps you can address it. Did you genuinely hurt him or drive him away unknowingly? Is an apology in order? If he cares about you but has lost interest due to the crappy way you have been treating him, then recognize that and change it. Have a brief but heartfelt talk with him, then back off. Work on changing your behavior permamently. Don't chase him. If he makes moves toward you after some time, then great. Perhaps the two of you could reunite. Make sure not to revert back to your old ways if you are given a second chance.
On the other hand, there may not be a thing you did wrong to chase your guy away. He may be immature and not ready for commitment. He may just not be that into you. He may have met someone else. He might have goals, such as traveling the world or finishing college, that would interfere with having a relationship with you. Don't assume you've done anything bad or wrong to have chased him away. Recognize the truth in the situation. If the relationship is doomed for any of the above reasons I described, accept it and move on.
Work on your physical self. Unfortunately, most guys are attracted to women physically before anything else. The better you look, the better chance you'll have of winning him back. This doesn't always work, but it's worth a shot. Keep in mind that some of the most beautiful beauties in the world (Christie Brinkley, Halle Berry, etc.) have had men cheat on them or do other horrible things to them. Beauty will not shield you from bad guys. But if you have let yourself go, start taking better care of yourself. Even if your ex coesn't notice, it will make it easier for you to meet someone better for you in the future.
If and when he does contact you (which he probably will), refrain from sounding negative in any way. Don't sound accusing or hurt or angry. Don't act overly emotional, either. Show no hint of desperation. Put yourself in his shoes, and make him feel glad he called.