WannaCry attack: What happened to my computer and how do I recover files?

WannaCry encrypts data files and ask users to pay a $300 ransom in bitcoins. The ransom note indicates the payment amount will be doubled after three days. If payment is not made after seven days, the encrypted files will be deleted. A look at how best one can protect oneself from ransomware:
 

• New ransomware variants appear on a regular basis. Always keep your security software up-to-date to protect yourself against them
• Keep your OS and other software updated. Software updates will frequently include patches for newly discovered security vulnerabilities that could be exploited by ransomware attacker
•  Email is one of the main