Iphone went black after using app

Been using iphone 4 for quite some years now. Then I got the battery replaced yesterday coz of battery problem. Today, I was chatting using whatsapp and the screen went black. The thing is, the way it went black is kinda bizarre for me. It started with the freeze of the app, then it become whitescreen starting from the middle. Then it went black, still start from middle and spread outward till the whole screen was black. Tried the home+power button, didnt work. Tried charging/plugging it into a pc, the pc wont detect it. Any solutions guys..??