Relaced screen assembly, now have a "grid" on the screen.

This is often the case when either the display connections are not properly seated, the display itself is damaged somehow (Doesn't necessitate a break to the LCD or Glass), or unfortunately the possibility of a bad Logic Board.

I'd recommend double checking the Display connections to the board (You can test this by pressing with Some but not Hulk-Like force on the right top of the display) and see if that makes a difference. An odd point to note, but did any of the stand offs for the display cowling screws seem loose, or did they come off when removing the cowling? I've seen this occur and cause a grounding issue before.

My guess is Bum display, Or possibly a bad Logic Board.

To clarify the issue causing you to replace the display, the phone would turn on, but you would get no Image? Or would it turn on with an image, but simply go black displaying nothing?

I had this problem after a screen replacement. I resolved it by double checking that the display connectors were seated, I also found I had mixed up the screws that hold down the metal cover over the display connectors, there are 3 different sized screws and they must be put back in the correct spots or they will not keep the cover plate snug.