I recommend inspecting the LCD cables for damage. They are fragile cables and can be damaged during reassembly (e.g., slight tear, pinched cable, damaged pin, etc.).
1. Open the device up and disconnect the battery from the mother board.
2. Disconnect the screen from the mother board and check the connectors for any indication of bent pins, missing pins, etc.
3. Take a microfiber rag (like you use for cleaning screens) or a Q-Tip and gently clean the connectors on the screen and on the mother board.
4. Reconnect the screen making sure to align the pins correctly and that the screen is secure.
5. Reconnect the battery and power on the device after the LCD has been reinstalled.
Finally, reinstall the display assembly. Keep in mind that connectors can become loose or dislodge completely during reassembly, which can easily be causing the display to not turn on. For example, if the battery is not seated firmly against the rear case during reassembly the battery can push on the display components and cables.
If the original display and motherboard are installed and your problem still persists, try swapping out each display and each motherboard to try all possible combinations. If still no joy, the problem most likely involves other component(s).
I hope this helps!