Hey there, mrbobbo,
According to the information outlined in Restore your reminders on iCloud.com - Apple Support, upgraded Reminders cannot be recovered. If you're not using upgraded Reminders, the steps outlined in that resource will guide you through recovery steps.
If you are using upgraded Reminders, is there a possibility the list was saved to your iPhone, not syncing with iCloud? If so, you might be able to recover the missing list by restoring a backup that was made while the list was still on your phone, as it would have been included in the backup file. You can restore a backup using the guidance here: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support. Restoring a backup does replace everything currently on the phone with only what was on it at the time the backup was created. To avoid losing newer data, please make a current backup before restoring an old one: How to back up your iPhone, iPad, and iPod touch - Apple Support
Should you like to see additional features available in future iCloud updates, let Apple know directly here: Feedback - iCloud - Apple
Best.