This video will explain how the Petya Ransomware affects your computer. This ransomware is different than the others that are currently being distributed as it targets the master boot record of the victim’s hard drive. This allows it to show a ransom note before Windows starts.
Note that restoring the MBR will not fix this as the hard drive will still be encrypted. Please wait till Petya is analyzed further in the event that it can be decrypted.