Screen/display remains black - though phone is working!

I (perhaps foolishly!) decided to replace the battery and back cover on my trustworthy 3GS. I did this circa two-three months ago. It went without hitch apart from the camera not working (I couldn't get the camera in the holder, with connecter attached to logic board so gave up).

Anyway, I decided I needed the camera back (as I use it a lot for work as I was sick of batteries going on normal camera). So I took it apart again.

Now my screen won't work. It is black, and you cannot see anything on it under a bright light. However the alarm clock is still going off and it makes a noise when plugged to a charger/laptop. I can still add stuff etc through the mac to the iPhone, and back it up etc - just nothing on the actual screen of the phone!

I have tried the hard reset numerous times, I have taken it apart several times to check the connections and to take the logic board off to "drain" before reconnecting (I couldn't turn it off before taking apart and someone suggested on here that there may be a safety feature if disconnected while it was still on).

Any ideas...? Totally baffled.