My iphone keeps on flickering

Hi! I got my Iphone 4 (it's not completely new because it used to be my parents but it's still new).

I was listening to music than i went to camera and took a picture when i clicked to see the picture, my Iphone stopped and there were two lines going down and at the bottom there was one going across and the screen was white. And it stayed like that for about 30 minutes. Than i pressed the home button and the switch off button and it turned on but while it was turning on it started flickering. So i entered my password and it's still flickering. I tried everything with the brightness; Turning auto brightness off. And i have tried restarting it but the flickering does not go away!! Please HELP ME!! I NEED AN URGENT REPLY!

P.S my Iphone is iOS 7.1.2