The most common reason for this is that you changed your password. This can also happen if you've been inactive for a period of over 30 days. In all cases, what you'll want to do is
1) Go to Social Networks Auto Poster from your Dashboard
2) Click to Show Settings for the social network you are experiencing problems with
3) Look for the link to Re-Authorize your Account and follow that process through to the end. You may want to make sure you are logged into your social media accounts before you click the link (although if not, you'll be prompted to log in
4) When you're done, click to Submit a Test Post and see if things are now working.