You can recover completed tasks in Kolleno by clicking into the "View archives" button in the top right hand side.
You can recover responded, dismissed or snoozed tasks.
Once you have done that, you will see three dots on the right hand side of any completed task, near the Status. Click "Recover" to make the task Live again in your Collector Tasks.
Be sure to click "Back to live folder" in the top right hand side to return to your live Collector Tasks.