If you've run out of credits half way through a Campaign, not to worry!
You'll first need to top-up your credits via the Buy Credits link in Admin > SMS & Email.
Once you've topped up, you can head on over to Marketing > Campaign Report.
Specify your date range for when the Campaign was sent, and locate the Campaign that you're looking to re-send.
Once you've clicked Re-Send, you can choose to re-send the Campaign to everyone, or Failed Recipients Only.
If you want your credits to automatically top-up when they're low, click here to learn how.