Server not responding?

I know next to nothing about computers, so I really need simple answers here
When ever my friend tries to join my server (I just click create multiplayer) it says “server not responding” and the same thing happens when I try to join his. What do I need to do so we can play together?

We got it to work a couple days ago. Thought that might be an importnat thing to mention.

Is he on the same network as you? those servers only work LAN (Except if you’ve went through a butthurting method to fix it)

I’m not sure?