If you have actually blocked somebody inadvertently or altered your mind about the block, though, it is also reversible. You can change your Privacy settings to unblock and allow somebody back into your Facebook world.
How To Unblock A Person On Facebook
1. Click the down arrow next to "Account" on top 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 wish to unclog. A list of the people you have actually obstructed is found under the Block Users section.
Idea
Uncloging somebody does not instantly put that person back in your good friend list. You need to include the newly unblocked individual as a buddy 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 To Unblock A Person On Facebook with your friends.