An individual going by the twitter handle leostone was able to create an algorithm that can generate the password used to decrypt a Petya encrypted computer. Leostone has setup a web site (you can use this site if the other one is down) that a victim can use to generate the key once they provide some information from the infected drive. 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 the boot code, or C: drive, for your computer.
Source: https://www.bleepingcomputer.com/news/security/petya-ransomwares-encryption-defeated-and-password-generator-released/