When common ground is lost, friendships deteriorate fast. Colleagues can be no fun outside the workplace, and after graduation students lose touch. It happens all the time and to everyone. The fact that this person doesn't call you back probably doesn't have anything to do with you. It's just that now school is done with, people would like to move on and meet new people.
So, allow yourself to feel bad. Perhaps take part in a good-bye ritual too, so you can move on. You could for instance write down everything you think or feel or like about this person and then fold the paper you're writing on into a paper air plane or boat - and then let go of all your feelings about this person, by letting go your paper plane or boat. Sounds silly, maybe - and you might have to do it again in a week or year or so, depending how much you've attached yourself to the person you mentioned.
Oh, I forgot to share a good-bye ritual I sometime use - and it really works for me. When I hurt inside from missing someone I loved to pieces, I go outside, lean to a wall, tree or car and stare at the clouds that pass me by. I then proceed to tell the clouds what I feel and let the clouds carry my emotions wherever they please. When I focus on letting out my deeper, underlying feelings, this technique works for me. What doesn't seem to help is to get angry or focus on how much I hate myself or others. To release some of the pressure I have to say things like: I miss X so much, I loved X so much and so on.
