Depending on how the customer(s) were lost, your strategy will vary. Start by identifying why they were lost in the first place - was the mistake the result of a deficiency in quality, service, pricing, or something else? Regardless, this will be a time for your customer service to shine. You might pursue them again as if they were a first time customer, offering introductory sign-up rates or freebies to regain their interest. Make sure to take note of which strategies work in these instances so that you can develop a protocol and revert to these strategies in the future if/when this scenario happens again.