Well, I can give you advice on what I do, but nothing ever works for everybody so take what you will.
You could start off by saying like “hey haven’t talked in a while. How have you been?”
If you guys said goodbye because of what you did or said, then apologize after the greetings and tell them that you miss there friendship or whatever the relationship.
If it was the other person(s) at fault for the goodbye, then if you forgive them(if you do) and tell them that you want things to go back to the way they were before.
If all goes well y’all will start talking again and maybe even hanging out.
I hope this helps you some for whatever your going through.