Open your Facebook application
Click the menu bar on the bottom-right corner of the screen (iPhone) or on the top-right corner of the screen (Android)
Scroll down to find and select Settings. This step is non-existent on Android devices
Click Account Settings
Select Blocking
A list of users you’ve blocked is displayed here with an Unblock button to the right of each user.
Click the Unblock button to unblock any user you wish to not block anymore
At this point, you’ll be asked if you’re sure you want to unblock the user, and also given the “Unblock” and “Cancel” option buttons
Click the Unblock button
The user should be successfully unblocked.
Desktop
Sign in to your Facebook account by visiting the Facebook website and entering your sign in details
Click the downward facing arrow on the top right corner
Select Settings from the drop-down menu
Click Blocking on the left side of your screen
A list of blocked users will appear close to the “Block users” area on your screen’s mid-section. Each blocked user has an “Unblock” button to its right.
Click the Unblock button
You’ll be asked if you’re sure you want to unblock the user and provided with the “Cancel” and “Confirm” option buttons on the bottom right corner.
Click the Confirm button
Before you proceed on Facebook to unblock that your blocked friend, bear in mind that you do not automatically become friends with unblocked users. To be friends once again, you have to send them a friend request.
Another thing is that, if you mistakenly unblock a user you do not want to unblock, you’ll have to wait 48 hours to block the user again.