Unless you did anything to offend them (which it doesn't sound like you did), I highly doubt that the reason they stopped talking to you has anything to do with you. As you know, friendship requires effort in both directions, and many people lose the drive to put forth that effort after a certain period of time passes. So it's not that you did something wrong, it's just that you may benefit from concentrating on finding some more stable friends who are willing to make a commitment to be there in good times and in bad.