Once a campaign has been sent out it disappears from your campaign list. However this doesn't mean that you can't use it again. To do this, simply start a new campaign and then in Step 3, where you pick the template that you want to use, you have the option in an extra tab for "Past Emails Sent". This allows you to see every email that has been sent previously and then select which email template you wish to recycle. You can then edit the email further if you wish.