<human>: Can you condense the text into three sentences or less?

This happened like two week before. My cousin was trying to format his computer and upgrade to windows 8 from XP. But due to hard disk problem, it didn't install anything above windows XP. So, he asked me if there was any thing that was wrong with the computer. He shared me some technical errors that appeared at the screen, which I immediately recognized as hard disk problem.

So I told him how to repair it. He didn't understand what I was saying, so I decided to demonstrate what to do. He was copying every steps that I did, so he wouldn't have a problem later on. There was this one step while solving this problem, that I had to open a program called "DISKPART", and there is a feature named "CLEAN" which completely formats the hard disk.  So, I said that you just have to type clean there (and i typed clean too) and press enter (which I was very careful not to press). He started reviewing the steps he had written to make sure everything was fine. When it was all right, I just said "Yeah!! that's it, and at last just press enter." and I actually pressed enter at that time. I immediately realised that clean command was still typed there and my hard disk was starting to format. I removed the laptop's battery as soon as possible, and waited for sometime before keeping it in again and restarted my pc. Unfortunately, all the data had been lost. It was gone beyond recovery. Well, i thought it was not a big deal. but when my parents came home, they were surprised to see every documents they had on pc deleted.

they asked what happened and I told them the truth. They didn't do anything to me(fortunately) but I came to know yesterday that the laptop had my mom's only digital copy of her Masters Degree Thesis, and my family suffered a heavy loss of around $5000 work as my father lost some valuable office documents he had prepared for around a month. I am just glad that my parents are not giving me hell right now.
<bot>: Cousin had problem with hard disk, showed him how to completely format it, also formatted my own hard disk too. Loss of very very important data.