Ransomware is malware that locks your computer and mobile devices or encrypts your electronic files, then offers a way to get it back - you pay for a digital key to unlock your files, which you may or may not receive. All your data is locked up, held for ransom.
The No More Ransom Project offers infected users the chance to get their data back using their free tools.

Start by using Crypto Sheriff to define the type of ransomware affecting your device. Once the type of malware that has encrypted the files has been identified, it has to be removed using anti-virus programs before any files can be unlocked, otherwise it will keep encrypting data. Once the malware has been identified and removed, No More Ransom has free decryption tools (from four variants of ransomware) and a how-to guide for each decryptor to help unlock encrypted files.

The site has a handy Ransomware Q&A, prevention advice, and links to report a crime in Europe, Netherlands and the USA (ransomware is considered a crime and they urge you to report it).