Screen blackens whenever lens opens, hence can't take pictures

Camera turns on but does not take pictures. Screen works fine for displaying menus using buttons, and calling up images. But when I EXTEND LENS for picture-taking, screen turns black. Pressing silver button does nothing. I simply cannot see any images through the lens on my screen. I can press a back button to return to Playback Mode and screen works fine as long as I am not looking at data derived from what would ordinarily come through the lens. Unfortunately, I did not have manual for camera, as I do now, and I missed checking some back button options before I took camera apart. I am putting the camera back together now on the assumption that the camera was in ECCO mode. Any suggestions appreciated.