My iPhone 3GS 6.1.6 can't be detected by iTunes, computer, and other softwares. I've already tried using another USB and another computer but it didn't work. The screen of my phone is completely black. Whenever I try entering DFU or recovery mode, it won't work. I tried charging the phone but nothing appears on the screen. What should I do? It is a second-hand phone and before these events happened, I've jailbroken this phone unsuccessfully. Please, help!