If you have actually blocked someone accidentally or changed 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.
Unblocking Someone On Facebook
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 "Unblock" link beside the name of the individual you desire to unclog. A list of individuals you've blocked is found under the Block Users section.
Pointer
Uncloging someone does not automatically put that person back in your pal list. You must include the recently unblocked individual as a friend and wait for them to validate the demand.
And one more thing please don’t forget to share this awesome trick to use the Unblocking Someone On Facebook with your friends.