It sounds like there is simply a lack of communication between the two of you because of some unsaid reasons. The reasons he may have cut off communication to you could be many and you will never really know unless he tells you. Here is something you may want to try to see what is going on...
Write him an email or a letter and in the letter let him know how you are feeling. Since you do not want to get in between him and his girlfriend make sure you say in the letter that you simply miss the friendship that they shared. You also make some guesses in the email as to why things have become awkward between you, but simply offer them as possibilities and see if he can clarify.
Or if you wanted to make it short and sweet without getting too much into things you could even write something as simple as, "Things between us seem distant and awkward. This is sad to me because I miss the friendship we used to have. Did I do something? Hope to talk soon. Feel free to write me back too." This is short, sweet and lets him know how you feel.
The reason you may not want to do this is because you do admit you have a crush on him. When we are around people who are our friends that we have more than just friendly feelings for, it can be hard to keep up the good boundaries. It may actually hurt worse if you become friends with him again and then get closer to something that you know you cannot have. Proceed with caution and make sure you are always aware of how you are feeling. If it is getting too close for comfort, perhaps a little distance would actually be a healthy thing.
Please take care!