Htc 826 no display after power button replacement

My dear experts

htc 826 dual sim came for repair of power button (on/off switch).

Before replacing the on/off switch lcd is working good.

But after replacement of on/off switch lcd is blank with no lighting but the set is ok with usb recognisation on pc and when power button is pressed it is giving vibration and the mother seems to be ok but no display working.

I changed 3 new displays but no display working and no lighting on lcd.

So dear all of my experts please suggest me the proper solution.

What would be the reason for this no display