Content Sync will retry all failed updates multiple times in a timeframe of 24h to automatically recover from temporary failures. If you are unsure about how to troubleshoot issues, please checkout What if a synchronization / update fails.
After 24h the update will be marked as failed. Once you have resolved the issue you can restart the update by clicking “Retry” when viewing the update e.g. in the “Content Cloud” tab at the content or the “Synchronize Content” tab in the Content Sync settings pages. If you have a lot of failures from one specific issue and don’t want to retry them all manually you can also use a “Retry failed” mass update to restart all updates that failed before.