How to Resume a Friendship With a Long Lost Friend
To resume a friendship with a long lost friend, start by reaching out to them in a low key way such as by text, email, or instant messaging. If you haven't heard back after a week, call them and leave a light-hearted voicemail, saying something like “Hey…time has flown by and it's been years since we talked…but I miss you!” Once you've heard back from them, reconnect by sharing a laugh about a good memory or catching up on what's new in your lives, and then maintain contact regularly.
Find your friend. First you have to locate your friend if you've lost touch. If you are still have their contact information or mutual acquaintances, this should be fairly easy. If you have lost track of them altogether, you might want to look into how to find someone. Initiate contact. Reaching out is the hardest part. Try to start by using a written medium that provides a delivery confirmation. Email, texting, and instant messaging are a great way to initiate contact in a low-key way. Striking up a normal conversation as if nothing is unusual is one way to start. Another way to start is by addressing the long absence and expressing regret. Give them time to process. Just because you are ready to connect now doesn't mean that it was the first thing on their mind. While you took the first step, they may also have to take some time to evaluate how they feel about the situation. Don't freak out if they don't get back to you right away. Give it several days to a week. Call them. If you haven't heard back in a week, you have a decision to make. You can either accept that they don't want to reconnect and give up, or you can try to connect by calling. Calling is much more personal and you can convey empathy, which some people respond to better. Just don't be shocked if your desire to reconnect is not shared. If you are worried about rejection, you can try calling at a time when they aren't going to be available and leaving a message. Try to keep your message lighthearted but sincere. For instance, "Hey... time has flown by and its been like, years since we talked... but I miss you!" Consider writing your message down or taking notes in advance. Allow time for your friend to trust you again. After a long absence, it can be hard for people to reattach. Sometimes, it takes a lot of effort on your part, and you may feel like you are the one working to restore your relationship all alone. Give it time. In some relationships, it is easy to reconnect quickly. The absence will be water under the bridge. In other relationships, it may take more time. Particularly if the parting was acrimonious. Remember old times. Especially in the beginning, it can help a lot to remember what made the two of you friends in the first place. Sharing a laugh about a good memory that the two of you have is a great way to reconnect. Catch up on what is new. Since some time has passed since the two of you were last close, odds are some things have changed. Catch up on their life and be open to sharing what has changed about yours. The two of you have probably both grown as people, and may have plenty to learn about each other. Pursue common interests. Just as you would with anyone you wanted to be friends with, pursue common interests. Whether the two of you have an old love of the same sports team or share the new hobby of home-brewing, you should find common ground and interests to keep your friendship strong. Connect often. Especially early on, you need work hard to reestablish the friendship. Initially, you may feel as though you are doing most of the "work" in the friendship. Hopefully things will get easier with time. Of course, don't pressure them too much. If after several meetings you still feel like you are the one taking all the initiative, you may be smothering them. Slow down. Make contact regularly. Get into a rhythm you can both trust as the weeks and months go on. Talk about and become involved in your friend's life and them become involved in yours. Sharing your lives regularly will keep your friendship strong once you've found one another again. Come to grips with the situation. Sometimes, you are the reason why your friendship dissolved. Sometimes, it is more due to extenuating circumstances. At times, it can be helpful to understand and deal with the cause of the friendship ending in order to strengthen it and prevent the issue from occurring again. Figure out why you fell out of touch. There are plenty of both good and bad reasons why people fall out of touch. If you want to rekindle the friendship, you may need to first come to grips with why it fell apart in the first place. There are some common reasons for falling out of touch. Romantic entanglement. Either between the friend you fell out of touch with, or over a mutual acquaintance. Distance. Separating by moving away for work or college causes many friendships to wither. A dispute. Perhaps there was a fight that you never reconciled over. A stressful situation. A shocking situation such as a death or illness in the family that happens to one friend can cause them to withdraw. Betrayal. Whether real or perceived, nothing hurts worse than feeling stabbed in the back by someone you trusted. Take responsibility for your part in losing touch. While many friendships fall apart naturally, sometimes there is personal responsibility to take. If you can't take responsibility for your part in falling out of touch, it may be difficult for you to be able to reconnect with the other person. If you still insist that it is all their fault, you probably aren't ready to move forward. If they get the sense that you blame them, they may have a hard time opening up to you again. Learn from your mistakes. Now that you've confronted the reason the friendship dissolved in the first place, you'll be better prepared to avoid that in the future. Hopefully you and your friend have many enjoyable years ahead of you! 
Find your friend. Initiate contact. Give them time to process. Call them. Allow time for your friend to trust you again. Remember old times. Catch up on what is new. Pursue common interests. Connect often. Make contact regularly. Come to grips with the situation. Figure out why you fell out of touch. Take responsibility for your part in losing touch. Learn from your mistakes. 