Can somebody please tell me how can I un-block my friend to get his messages in my mailbox once again?

Go to your Posteingang and select the Blocked Users section. There should be a green Unblock button to the right of your friend's username. Click on that button and you've unblocked him.

If you're on Fanpop mobile site, then Du should go to your mailbox using the URL with a tag /messages/trash Du can find all of the messages sent to Du Von your friend right there, amongst your trash mail. All Du gotta do now is simply to choose your friend's message & tap Undelete selected messages button beneath the list. That's it. It's as simple as that! Your friend is now unblocked & all of his messages will go to your Posteingang instead of trash. Hope it helps. It works for Android 6.0 oder higher. I'm not sure about the other mobile OS's though. Du forgot to mention what kind of a mobile OS do Du actually use, if you're using the Fanpop mobile site in the first place. Du should be Mehr specific in the future when it comes to Mehr detailed informations, so that we can all provide Du with an adequate & satisfactory answer Du deserve.