please excuse, I am new to iPhone, as I repair it for a friend.
It's an 4S from USA that some other friend of him wanted to unlock for germany.
So I don't know what he did exactly, but:
When I restart by holding home+power, it restarts like expected. The apple appears for 2 minutes, then the (respring?) turns 180 degree and then stops.
There's also no way to get it into DFU mode, it boots no matter if I hold the home button.
Also when I hold vol+, it won't boot.
I'm out of ideas to get this to boot.