Ransomware is a new virus came out recently in internet world.This virus also referred as wannacry. This virus has significant potential that your computer/server will be encrypted in fraction of second.Complete control of the system will be goes to the hackers and they are asking to pay money to unlock your system.
Attackers choose email gateway as their transmission media,So do not open an email from unknown source.Also do not open email attachment or links given in such kind of emails.Some time there could be an option they have change the from address as like your one of the trusted site. So in this case not only rely on the name, just mouse over the link to identify the URL it going to redirect.
This virus hit severely in US and UK recently due to this some of the govt services also get bolcked.
So now how to protect your system?
Keep your system up-to date with your window's update and if your running any anti virus keep the anti virus software up to date in your machine, so that it can detect such kind of virus.Very important thing is do not attempt to open unknown email attachment/links.