How can you get your files back?
F-Secure advises against paying the ransom. While doing so is one way to regain control of your computer and data, the real remediation begins before you ever get hit – by taking regular backups. That way, if you do get attacked, you can relax – and restore everything from the backups. Furthermore, even though most ransomware has returned control, this may not always be the case. You may end up paying and still being left without control.
If your files have been hijacked and you don't have backups, it's worth going online and seeing if there is a decryption tool for the ransomware that you've been hit with. This list is a good start, although decryption tools are typically only available for early versions. And keep in mind that attackers update their approach and use ransomware that doesn't have a decryption tool available.
You also might find it useful to share your situation on a help forum like Bleeping Computer, where there are threads to help with Locky, TeslaCrypt, CryptoWall, Petya, CryptXXX, Locker, and many others. It is also recommended that you report the crime to the relevant authorities, typically the police.