Ransomware

Ransomware is computer malware that installs covertly on a victim's device (e.g., computer, smartphone, wearable device) and that either mounts the cryptoviral extortion attack from cryptovirology that holds the victim's data hostage, or mounts a cryptovirology leakware attack that threatens to publish the victim's data, until a ransom is paid. Simple ransomware may lock the system in a way which is not difficult for a knowledgeable person to reverse, and display a message requesting payment to unlock it. More advanced malware encrypts the victim's files, making them inaccessible, and demands a ransom payment to decrypt them.

Ransomware is created by scammers who are highly knowledgeable in computer programming. It can enter your PC through an email attachment or through your browser if you happen to visit a website that is infected with this type of malware. It can also access your PC via your network.
It's obvious when your device has been infected with ransomware, since you most likely won't be able to access your computer.
Make use of your antivirus software's ransomware removal tool, which should scan for and wipe out any ransomware attempts found on your computer.

You can download free anti-ransomware tools below. These tools will remove ransomware viruses from your computer and decrypt any files that have been encrypted in the attack. They’ll also inform you about the types of ransomware and show you what they look like.

Free ransomware removal tools

Alcatraz Locker
Apocalypse
BadBlock
Bart
Crypt888
CryptoMix (Offline)
CrySiS
FindZip
Globe
HiddenTear
Jigsaw
Legion
NoobCrypt
Stampado
SZFLocker
TeslaCrypt

Make sure all software on your computer is up to date, including your operating system, browser and any toolbar plug-ins you use.
Ensure that your antivirus software and firewall protection is up to date.