LCD digitizer assembly doesnt work after replacement (green lines)?

I saw your video for replacing the LCD-digitizer of my nexus 4 phone, and followed the steps carefully. After I placed the battery, I checked to see if the phone turns ON and the display is working before I complete the last step of putting the back coverplate. I successfully switched it ON, saw a perfectly working screen, and browsed through my apps. To complete the assembly, I put the back cover by gently pressing the frame along the edges, but suddenly I saw some yellow green lines in the middle of the screen. I switched it off and again switched in ON, but now the entire screen had light with no display.

To fix this, I tried again checking all connections but no luck. What could be the reason for the sudden loss of display when it worked perfectly fine moments ago? Is there a possibility that I damaged the LCD while putting the back cover on the phone?

I am also not sure about one of the ribbon connectors of the digitizer. There are 2 big ones which are prominent and has ports to connect. However, there is a little third one which also has to come through the gap but doesn’t connect anywhere. Has anyone also experienced this while replacing the screen? Any help will be appreciated.