If you have obstructed someone mistakenly or altered your mind about the block, however, it is likewise reversible. You can alter your Personal privacy settings to unblock and permit someone back into your Facebook world.
Unblock People On Facebook
1. Click the down arrow beside "Account" at the 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 "Unblock" link next to the name of the person you desire to unblock. A list of individuals you have actually blocked is found under the Block Users area.
Tip
Unblocking somebody does not instantly put that person back in your buddy list. You need to add the newly unblocked individual as a friend and await them to confirm the demand.
And one more thing please don’t forget to share this awesome trick to use the Unblock People On Facebook with your friends.