iPhone reset fails after Front Panel swap. Display only has backlight.

I have changed the Touch Screen and the Front Panel glass of the iPhone. After a reset the iPhone displayed everything and did a normal reboot, only the Touchscreen was working not that well, wrong calibration and some parts does't accepted my finger tips. So I bought a new Touchscreen and replaced the other one. After that I tried a reset with Home and Sleep Button. I saw the Display turn black (The backlight was gone), but then the iPhone stayed of and there was no recognition of buttons pressed anymore. The flax cables are all right, the plugs are check a few times and the buttons are all working. The circuit board was disconnected from battery while repair and after repair it also powers on (only backlight) iTunes is recognizing the iPhone and the speakers do the connected sound, the battery is loaded about 80%. iOS Firmware is 4.0.2, but it should be a hardware problem.