This is done be hackers sending an email to you with an attachment. The attachment is a Microsoft that will change all of your important Windows files with an extension of ".locky" and decrypt all of those files so you cannot view them. This is done via a MS document macro which is nothing new. This has been done many years ago and seems to return now and again.
Read more about it here:
http://www.enigmasoftware.com/lockyfileextensionransomware-removal/ (http://www.enigmasoftware.com/lockyfileextensionransomware-removal/)
http://www.2-spyware.com/remove-locky-virus.html (http://www.2-spyware.com/remove-locky-virus.html)