[AC Question]

Hi, I was just playing around with my LG G3, (I'm using Nova Launcher) and I found "Guest Mode". I turned it on, and it changed the screen to the default lockscreen, and then the screen just went completely blank. The led flashes blue if I press the power button, and I know I can turn the screen on and off because I can see the "Retro TV fade out effect" when I press the power button, but it is completely blank. I can't turn the phone off via the power button as nothing come up, it's completely blank. I would like to avoid anything which involves me losing all my files and data. My carrier is O2 and I got it in April 2015. Please, help me!!!