An unhappy Customer is an incredibly valuable asset. Their feedback is a reality check for your company.
Statistics show that A typical unhappy customer tells 10 people about his problem.
We live in a world of virtually unlimited choices and fierce competition. They could have just taken their business elsewhere;
But the fact that they have invested their time to contact you, consider it as and indirect invitation to engage in improvement of your service or product.
Thank Them
Apologize
Offer a solution
Follow up

Remember keeping your Customers happy improves the bottom line.