If you completely blocked the application, you’ll need to go to Account – Privacy Settings – Block List. There will be an option on the privacy settings page, where it says Block List, that says “Edit your lists of blocked people and applications.” you click on that, and if Farmville is in the list, just click on “Unblock” next to that, and everything should be fine.
Source(s):
Daily Facebook user, and help from a facebook group called “how to unblock a blocked application”