Sorry for the scare, this isn't a real ransomware attack. Think of it as an intervention - you really need to watch what you click!
The best time to stop ransomware is before it starts. Make sure your important files are backed up in the cloud or to external storage (USB drive, SD drive, etc.). This way you can recover them if you get infected.
Keep your software up to date. Ransomware usually relies on software bugs to attack your computer. By updating your software, you can install bug fixes and reduce the chances for ransomware to strike.
Be safe online. Avoid suspicious sites and links (like the one you clicked to get here!), "look for the lock" in the browser's URL bar when browsing, don't open spam or suspicious attachments, don't share your passwords with anyone, and so on.
Ransomware could encrypt all your files, rendering your pictures, videos, documents, and work files useless. If it's worth something to you, BACK IT UP! The interval at which you back up your files should be dictated by how often they change, and how much of those changes you can afford to lose. The most important thing you can do to fight ransomware is to...