assuming you still get the display over to the external monitor, i have a couple possibilities
1) cable disconnecting while mounting the screen: Open up the device, and make sure the cable stayed connected while installing the screen. They use a pretty tough tape to keep it installed, and sometimes it doesnt hold after tampering.
2) screen they sent you was faulty: Check the warranty policy of your screen, and see if they can send you a second one.
3) device was operated on with battery in: If the device was operated on with the battery in, the power source could have fried out the LCD or the GPU. or sparks could have melted the mounting plastic on the connectors
4) connector on either the cable or screen got damaged: make sure there is absolutely zero physical damage to the connectors on both parts. (broken corners, bent pins, etc) a microscope comes in handy for this
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