what did i leave out?

i replaced the flex cables in 2 sony ericsson z520as. they both had totally white main screens but the exterior screen worked.after the cable installation both screens are black. i put the original cable back in to see if the white screen would return, but no, still black. i read on your forum that the connections must be reset??? is this true? how do you do that?? is there a step i am leaving out?