A formal letter I want to write to a notebook provider

<I just want to convert my thoughts into formal sentences. Please help me. I'd be very thankful if you could rewrite my entire letter. Thank you so much indeed>

Dear Sir

I purchased successfully a notebook from your own shop three days ago. The machine had worked very well until yesterday evening. I brought it inside my backpack and cycled about 20 minutes home. After getting back home I putted it on the table then powered it on but it won't boot any more, just black screen and no leds were lighted. I have checked the power supply and it worked fine. Everything outside the notebook don't look unusual or damaged at all.

Please tell me what should I do. Could I be able to return it back and get another the same one?

I would be very grateful for any helps you are able to provide.

Best regards

Hi, I am not sure purpose of this letter, if IELTS you need to start with Sir / Madam. Rest of the paragraph as Kevin said.

Dear Sir / Madam

[...]

I would be very grateful for any helps you are able to provide. - Ask Strongly and say what you will do if it is not replace

I would be very grateful if you could replace the notebook in one week, if not I insist on getting a refund of my full payment.

Best regards is a casual way of finishing, formal letter you have to finish with - Yours sincerely,