Ransomware is a type of phishing attack that is initiated when a user clicks on an infected link in an email or infected advertisement. Malware is then deployed on your computer and blocks you from accessing your files. You will then be instructed to pay a "ransom" hence the name to restore your files. The ransom is a scare tactic to force you to make a hasty decision.
Any computer user has the potential to get infected, especially if you aren't cautious about links in emails.
Take steps to prohibit this type of activity by taking note of the following:
Don't pay the ransom and stop using your computer. Retain the help of a reputable computer professional who can help you restore your files, advise you on paying the ransom and check your computer for further infection.
The American Bankers Association infographic on Ransomware is a great first step in arming yourself against this type of attack.