Facebook issues its tokens for 30 days. Each autopost renews the token. This message means that you either haven’t posted to Facebook for 30 days or Facebook has your token invalidated for some internal reasons. When you changed the Facebook password session has become invalidated as well.
The solution is the same, please to to the plugin Facebook settings and click “Re-Authorize this Facebook Account” link.
Other networks also have some expiration rules for tokens. Solution is the same – click “Re-Authorize this XXXXXXX Account” link.