After screen disassembly, display is dead?

Hello, I got a faulty iPhone 5s which looks not opened yet. It stucks on the apple logo and I got error 9 and 27 on iTunes (dfu mode and default).

So I decided to open the phone to see the battery apn. I followed the tutorials and it works fine. I loosed the screws and put them in the right holes. I ordered a new battery and after the battery arrived, I tried to turn the phone on but I saw nothing on the screen. iTunes recognized the phone.

So I opend the phone again and replaced the battery. After I closed the phone, there was again no screen displayed. iTunes recognized the phone again but now I get the error 2009. It means the cable is faulty but it isn't.

I don't know whats wrong with the phone. My next would be to replace the screen.

Any other suggestions?