On your Facebook homepage, on right side, you will see the sign of "Privacy Shortcuts" next to the 'Logout' button.
Then click the option “How do I stop someone from bothering me”
The click “View all blocked users”
Then click “Unblock” next to the name of the person you want to unblock
Note: This will not automatically add the person back to your friends list. If you want to become friends with this person again, you will need to send them a new friend request.