Nothing. You have been pranked. Your files are safe and you can safely close this page by exiting it from fullscreen. However the WannaCrypt malware is no joking matter. Please take some time to review some of the tips on how to keep yourself safe:
The security hole has been patched in March, update your version of Windows to stay safe
Modern malware can infect your computer with very little effort on your side. Make sure to always check the file extensions and don't click on any suspicious ads
Ransomware can't keep your files hostage if you have backup copies. It's recommended to also use Cloud storage with version control such as Google Drive or Dropbox
It is not enough that you know how to stay safe from ransomware, pass these tips on to your friends and family so they can take measures as well. We're all in this together after all.