OK, I think I see the issue! Your system was a Fusion Drive’d model with two drives the NVMe blade as well as a SATA HDD.
Correct me if I’m mistaken your system is responding you get the bong then just getting a blinking question mark in a folder icon or your system fully dead?
If you are getting the blinking question mark in a folder you’ll need to use a bootable external drive to enter into Disk Utility to reconfigure your drive. As the system is still expecting the Fusion Drive.
If the system is not responding then something didn’t get reconnected or was damaged in the process. Did you follow this guide? [guide|30529] the process of taking the display off is the most critical as it’s very easy to damage it or the cables.