A Polite Email 

Dear Asha,

I hope you be fine. I got home safely, thanks. I'm glad you have liked the chocolates. I passed a good time there although I had not had time to explore the campus and town much. I forgot it,  as the room was a little untidy I decided to clean it and I found a necklace and left it in the closet.
Firstly I'm afraid I do not know nothing about your computer. I did not touched it because I had my own laptop. I did not need to use yours, but I hope you could fix it the sooner for your homework.
And about the key, when I was there I took it and I forgot to put it back and brought it with me in my jeans pocket. I'm really very sorry about that but don't worry I'll post it back.
I hope you have cleared your doubts and I also would like to meet you again this next year.

Best wishes, Antonia.