All of my files suddenly encrypted while watching movies online. How can I decrypt all my files and folders ?

Sounds like an attack of cryptolocker, or similar, ransomware.
The best way is to clear your system and restore from backups.

Yep sounds like you got ransomware. Like Curly says I hope you have your data backed up. I wouldn't pay the ransom. Wipe your hard drive and reinstall the OS, applications, and data.