Hi! Thanks for your reply! Where do you think is the exact problem?

Hi! Thanks for your reply! I really can't access DFU mode. I do it correctly. My computer doesn't detect my iPhone 3GS. Even on other computers with iTunes.

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...