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.
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.