Backup you important files (Win key+E for Explorer)
Click the Power Option in the bottom right
HOLD SHIFT and click Restart (Continue to holed Shift until at a new screen with options)
Click Troubleshoot, then Reset PC
Select the option to Remove Everything and Re-install Windows (you can try the option that doesn't remove files, but you will still need to re-install any programs/games)
Sit back and wait
Import your data if necessary.