If you have actually obstructed someone accidentally or changed your mind about the block, though, it is likewise reversible. You can alter your Personal privacy settings to unclog and permit someone back into your Facebook world.
Unblock Friends 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 "Unclog" link next to the name of the person you desire to unblock. A list of the individuals you've blocked is found under the Block Users area.
Tip
Uncloging someone does not automatically put that person back in your buddy list. You must 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 Unblock Friends On Facebook with your friends.