There is a rare but altogether annoying issue that may present itself in your Drupal website from time to time, which while having several different potential causes can be usually resolved in a straightforward manner. The "Failed to Get Available Update Data" error is a state when you go to the available updates report in your Drupal website interface in order to see which modules have updates available, and your Drupal website is unable to look up the status. It will look something like the image below:
One of the more complicated (but less likely) explanations for this issue is if your server or the website cannot resolve to itself. This might happen if the web server is behind NAT, and the provider does not support full split DNS functionality.
If you are using Drush to retrieve updates, then a simple cause and solution would be to check if Drush itself needs to be updated. You can review the most common ways to update Drush (depending on your version) here: https://www.drupal.org/node/901828
However, by far the most common reason for this failed update status issue is if the "cache_updated" table needs to be cleared. This database table is a storage location for cached information about updates, so manually clearing it (if normal Drupal cache clearing does not do the trick) should fix the problem. To do this manually would involve accessing your database controls (perhaps with a tool like PHPMyAdmin) and using the "truncate" feature on the specified table. Note: only do this if you are comfortable working with databases, as accidentally deleting or "dropping" a necessary Drupal database table can cause your website to behave strangely or crash completely.