To use [the software creator'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.herokuapp.com/ site to generate the key.