If you have blocked somebody inadvertently or altered your mind about the block, however, it is likewise reversible. You can change your Personal privacy settings to unblock and permit somebody back into your Facebook world.
How Do I Unblock A Facebook Friend
1. Click the down arrow next to "Account" at the top right of any page on Facebook.
2. Click "Privacy Preferences" from the Account menu.
3. Click the "Edit Your Lists" link under the Block Lists heading. It is at the bottom center of the page.
4. Click the "Unblock" link next to the name of the individual you wish to unblock. A list of the individuals you've obstructed is discovered under the Block Users area.
Tip
Unblocking someone does not instantly put that person back in your good friend list. You need to include the freshly unblocked individual as a buddy and await them to validate the request.
And one more thing please don’t forget to share this awesome trick to use the How Do I Unblock A Facebook Friend with your friends.