To use Leostone's decryption tool you will need attach the Petya affected drive to another computer and extract specific data from it. The data that needs to be extracted is 512-bytes starting at sector 55 (0x37h) with an offset of 0 and the 8 byte nonce from sector 54 (0x36) offset: 33 (0x21). This data then needs to be converted to Base64 encoding and used on the https://petya-pay-no-ransom.he... site to generate the key.
Unfortunately, for many victims extracting this data is not an easy task. The good news is that Fabian Wosar created a special tool that can be used to easily extract this data. In order to use this tool, you need to take the encrypted drive from the affected computer and attach it to a Windows computer that is working properly. If your infected computer has multiple drives, you should only remove the the drive that is theboot drive, or C:\ drive, for your computer.