It is essential to thank the customer for bringing the problem to you since by thanking you will be able to create a relationship with them. A simple “thank you” for their patience and time waiting for you to proceed and resolve the problem will do the trick.
As you analyze and seek a solution, a good way is to break the problem into smaller parts, facilitating your work and searching for results. You can see what is at your reach and where you will need the help of third parties.
Remember to make every step of the process transparent, even if it is the simplest, like if you can solve it with a call or if there will be more complicated steps. This way, they will feel heard and more comfortable.
Source: Hubspot, How to Deal with Angry Customers