Why is my screen black?

I Bought this phone on eBay for 50 bucks and nothing was wrong with it only the glass needed some replacing and it turned on and everything then one day the screen that was broken suddenly went black-blue while I was in dfu mode or recovery mode then I had tried to take the screen apart and put it back on and it had vertical lines and then it went dim so I went on amazon and bought a new screen and I disassembled the old screen parts and put it onto the new display and I connected the screen to the phone with the device in dfu or recovery mode (so The phone was on while I was doing the repair)and Pressed

The power button and it’s completely black but it comes up on iTunes and says it needs to update and restore my phone bc of an error on the phone but iTunes itself ran into an error (the phone was still connected into computer) where it the iTunes would make a disconnecting sound and reconnecting sound but it was still plugged in. I need help ASAP (the home button cable part that connects to the board is broken also)