How to Restore a Lost Facebook Connection with Your Blog.

This is from Support:

Reconnecting Facebook

Sometimes, the connection between Facebook and might get disconnected. Not to worry, the following steps should get you up and running again quickly.

  1. Log into your blog. Go to your blog’s dashboard -> Settings -> Sharing and click the Disconnect from Facebook link.
  2. Reload the page and confirm the link now says Reconnect to Facebook.
  3. Log into your Facebook account. Click on the arrow next to Home on the upper-right side of the page. Then, go to Privacy Settings and then click ‘Edit your settings’ under ‘Apps and Websites’ at the bottom-left of the page. Remove by clicking the X in its row. (If there is no row, don’t worry, just skip this step).
  4. Log back into your blog. Go to your blog’s dashboard -> Settings -> Sharing and  follow these steps to reconnect.