Sometimes, for various reasons, a site that you have connected to your iControlWP loses its link to your iControlWP account.
The site will appear in your List of Sites as an "offline" site, or a site that is having communication issues (as shown below). It will also have a Status Message as "Broken Link".
To re-link a site with iControlWP, you'll first need to install and activate the latest version of the iControlWP plugin. You may also need to reset the plugin's link status.
Read here on how to reset the plugin's link status.
Then you just click the re-link button as highlighted above and enter the new Authentication Key in the dialogue window that appears.