When you block someone on Facebook, you're preventing them from contacting you or seeing your profile. If you've previously blocked someone and they sent you a friend request, you won't be able to see the request or accept it. To accept the request, you'll need to unblock the person first.
Facebook does not have a specific “unblock friend request” button. Instead, you need to unblock the person first. Once unblocked, you can send a new friend request. how to unblock a friend request on facebook