[Your Name, & Address here]
[Recipient Name & address here]
I am writing this letter to express my embarrassment, the [item/article name] you lent me has been – [lost/broken].
Having known you for your unconditioned helping nature for all these years, I am really feeling awful about it.
Now, I want to buy you a new replacement for the [item broke/lost] by me. Please tell me where you got it from, or you can tell me the amount to pay you for the loss.
I am really sorry for this mistake and, really regret any inconvenience it might have caused you.
I am waiting for your reply.