that's easy all you have to do is patch the system and if that don't work reload the game and if you need any nooby quest help im PowerpuffDropOut ^_~
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if you created it right then they have to know you multiplayer server address to get to your multiplayer server
Type in the IP address or Hamachi address of your friend's correctly or play the premium Minecraft. Myncraft1: Your friends has to be on (Hamachi) to connect to his server
Type "What is my IP" On google and copy/paste it in your direct connect box if you have your router port forwarded and a server program running on your PC
There could be a number of reasons. (A) he/she might be using the browser to try to connect. (B) if it's a public server they might be "whitelisted". (C) not sure bout this one but they may have a mod unsupported by the server. Hope this helps!
You cant he is gone from the game.