Usually viruses known as ransomware decrypt files when victims have paid a substantial fee.one variant of Power Worm destroys keys that could help recover any data that it scrambled. basically some dude messed up this virus when he tried to simplify the decryption process. They tried to make it use just one decryption key but mangled the process of generating it. As a result, there is no key created for the files it encrypts when it compromises a computer.
http://www.bbc.com/news/technology-34765484