Unfortunately this happens often. I've had 3 best friends that we've been friends for nearly 6 years and one day they just stop contact with me and it ended that quick. The older you get you usually tend to have better odds of keeping friends but not always the case. Don't become a loner cause of this though as it won't help and definitely won't help in the long term. Everyone needs friends. Just keep looking for new friends and eventually you'll find another best friend. Also I would like to add, have you tried talking to your friend and see why he abandoned you or do you already know?