If you have obstructed somebody unintentionally 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.
Unblock Facebook Friends
1. Click the down arrow beside "Account" on 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 "Unclog" link next to the name of the person you desire to unclog. A list of the people you've blocked is found under the Block Users section.
Idea
Uncloging someone does not automatically put that person back in your good friend list. You need to include the recently unblocked individual as a buddy and wait for them to verify the request.
And one more thing please don’t forget to share this awesome trick to use the Unblock Facebook Friends with your friends.