Yes, absolutely, you should address the complaint and take it seriously. There are two reasons you should take this feedback and these complaints seriously.
The first reason is that every ex-customer is a potential future customer. They recognise your brand, they know your business, and they certainly know how to contact you. Deal with their complaint in a sympathetic, considerate way and you’ve just turned an ex-customer into a repeat customer.
The second, more important reason is that every customer, past or present, is a potential publicity machine for your brand. This is never truer than with dissatisfied customers. So impress them with your professional response: they’ll tell their friends and you’ll gain credit,and new custom in reward for your good customer service.