Sir/Madam,
This email is in reference to the request made by me on date __/__/______ (date). As mentioned earlier, I have purchased ________ (product) from your __________ (store/website/any other).
I write to bring to your attention that the product is __________ (not working/defective/damaged) and It needs to be _________ (repaired/replaced) as soon as possible as it is under warranty. I have already requested you but there has been no response from your end.
I am not at all satisfied with your customer service and I hope that you will take immediate action regarding this matter. If needed, you can contact me at ____________ (contact details).
Thank you,
_____________ (Name),
_____________ (Contact details)