Warfare is a constant back and forth between offense and defense, and the same rules hold true for cyberwarfare. A hacker finds a weakness in a program, the developer patches the weakness, the hacker finds another weakness, the developer patches the weakness, and so on.
However, that formula got flipped in the case of the TeslaCrypt ransomware. It was a major problem last year, but then security companies figured out its weakness and were able to decrypt and recover people's files without paying hackers. Unfortunately, the hackers behind TeslaCrypt learned their lesson and it's back and stronger than before.
In the original version of TeslaCrypt, the ransomware stored the key for decrypting the files on the victim's computer. That let security companies find it and use it to unlock the files. The new version of TeslaCrypt, version 3.01, moves the key to the hacker's server and deletes it off the victim's computer. That means there's no way for the victim to get around the encryption.
If you get TeslaCrypt on your computer, you'll either have to pay to get your file back or wipe the computer and start over. You better hope you backed up your files. Although, the FBI is warning that new versions of ransomware are starting to seek out and destroy local backups, so make sure you have an off-site backup as well.
Unfortunately, ransomware threats are growing at a fast rate, so you're increasingly likely to encounter this threat. Your best bet is to keep it an arm's length with strong security software.