U must check the flex cable to the display, ( make sure u do disconnect and reconnect it with out battery) ,reinsert batt and then check.
U must check if u have damaged any flex cable or left any scratch marks on any circuit boards etc which may cause any damage to the signal/supply lines.( use a magnifying glass)
U must check for some damaged components like smd resistor,capacitor etc..
If nothing helps, gave it to authorized service center.
Sent from my NexusHD2 using xda premium