Your connectors may be loose. Disconnect and reconnect them. If that still doesn’t fix it, check the ribbon cables thoroughly, as they may be ripped. Even the smallest knick can completely kill the screen. If even after that the display still doesn’t work, check the gold contact pads on each connector.
If the display is still dead, power the device on and make sure you see the white backlight light up. Wait a while and see if the display actually displays anything. The device may just need a while to boot.
If none of this works, send some pictures of the connectors and the display. You may need to order a new one. I also have multiple spare displays if you would like to see if it’s just the display. If you would like one you can comment on my answer here.
The “white light” you are seeing on the underside of the display is the backlight for the screen so I am assuming that the logic board is working and the battery is working. [br]
I hope this was helpful - Jack