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

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