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

Go to your caixa de entrada 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 you should go to your mailbox using the url with a tag /messages/trash You can find all of the messages sent to you por your friend right there, amongst your trash mail. All you 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 caixa de entrada instead of trash. Hope it helps. It works for Android 6.0 or higher. I'm not sure about the other mobile OS's though. You forgot to mention what kind of a mobile OS do you actually use, if you're using the fanpop mobile site in the first place. You should be mais specific in the future when it comes to mais detailed informations, so that we can all provide you with an adequate & satisfactory answer you deserve.