If you have obstructed someone unintentionally or changed your mind about the block, though, it is likewise reversible. You can alter your Privacy settings to unclog and enable someone back into your Facebook world.
Facebook Unblock Someone
1. Click the down arrow next to "Account" at the leading 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 "Unclog" link next to the name of the person you wish to unblock. A list of the individuals you've obstructed is discovered under the Block Users section.
Pointer
Uncloging someone does not immediately put that person back in your buddy list. You need to include the recently unblocked individual as a good friend and await them to validate the request.
And one more thing please don’t forget to share this awesome trick to use the Facebook Unblock Someone with your friends.