If you have blocked someone mistakenly or altered your mind about the block, though, it is also reversible. You can change your Personal privacy settings to unblock and enable somebody back into your Facebook world.
How Do You Unblock Somebody On Facebook
1. Click the down arrow beside "Account" at the leading right of any page on Facebook.
2. Click "Personal 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 person you desire to unclog. A list of individuals you've blocked is discovered under the Block Users section.
Idea
Uncloging someone does not instantly put that person back in your pal list. You need to add the recently unblocked individual as a friend and wait for them to confirm the demand.
And one more thing please don’t forget to share this awesome trick to use the How Do You Unblock Somebody On Facebook with your friends.