Whether you mistakenly blocked someone or simply wish to let bygones be bygones, you can unblock a contact on Facebook and allow her to contact you again by clicking on Unblock next to that person's name in the Blocked Users window.
Log in to Facebook and click on the Padlock icon to open the Privacy menu. Select How Do I Stop Someone From Bothering Me? to expand that section.
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Select View All Blocked Users to open the Blocked Users window.
Click on the Unblock button next to the name of the contact you want to unblock.
Select Confirm to finish unblocking that contact.
Blocking a contact also prevents you from being able to interact with that person. Conversely, unblocking someone lets you perform certain actions, such as send a friend request to that person or send her a private message. Facebook does not notify users after someone either blocked or unblocked them. If you feel that this is appropriate, you may therefore want to send the person you unblocked a message to let her know she can add you as a friend or contact you again.
If you previously blocked someone who was your Facebook friend, unblocking that contact does not automatically add her to your friends list again. If you wish to do so, you can send a friend request to a contact you unblocked by visiting her profile and clicking on the "Add Friend" button. Unblocking a contact may allow that person to view your Timeline, send you a friend request or send you a private message, depending on your Facebook privacy settings. You can edit your privacy settings by opening the "Privacy" menu and selecting either "Who Can See My Stuff?" or "Who Can Contact Me?"