It's been three months or so since you graduated. You've found a job, you've found fulfilling hobbies (and if you haven't, that's okay, there's always time) and you sit down one night after work with a snack and start to wind down. You get on the internet, watch a show, write something, and some words make something click in your head, causing memories from your college and high school days to come rushing back. You remember the good times or the bad, how awkward you used to be, and the people who were around you at that time in your life. You wonder how they're doing, you haven't talked to them for a while, despite being pretty good friends back then. You should contact them. Even if it's just a message saying "Hey," the attempt at continuing contact will maintain the bridge between you and this other person.
"But what if they don't like me anymore?" Have you done anything to make them not like you? "Well, I haven't talked to them for a while." If they're a reasonable person, they won't care. "But what about-" Shh, shh, shh, stop talking, just shoot them the message.
"Now what if they don't respond?" Then they're probably busy. Try again by messaging them the next logical step in the conversation ("How are you?") if they haven't responded within 24 hours.
"But what if they're still angry at me from that one time?" Time heals all wounds. Unless you killed their mother or did something of similar magnitude, I doubt they're holding whatever it is against you. And if it was bad, there is always time to try and make it right.
"But why don't they contact me?" Because everyone is busy in different ways. They probably think of shooting you a message and then don't because they get distracted by yet another of life's endless drudges, or they just aren't the sort of person who thinks to contact others until they have a chance to meet face to face with them, or maybe they're like you and feel guilty about something that you buried a long time ago. In all honesty, regardless of who or how they are, they probably would enjoy hearing from you to break up the monotony and catch up for a few minutes.
"What if I just don't want to?" Then that's your decision, and odds are you have your reasons, whether they be silly ones or serious ones. So navigate away from the messenger, go back to whatever you were doing, and forget until the next time this feeling washes over you. Or you could just shoot a message and not have to be left wondering next time.
Keeping in contact isn't difficult and takes almost no time. Never assume that somebody you know doesn't want to talk, you can't know that unless you find out for yourself. It's okay to not talk to somebody for a while, but if you ever do, make sure that it isn't in order to solicit a favor unless you are in the most dire of straits. People don't mind if they haven't heard from you in a while, so don't worry about not talking to them every day, but make sure that they hear from you every now and then. If they didn't want to talk to you, they wouldn't have been your friend in the first place.