Screen remains black after screen replacement

Start with a simple hard reboot:

Press and hold home button + power button together for ~10 seconds. Then let go of home button, while still pressing power button.

A/ The iPad restarts normally,

B/ Restarts, but the Apple logo is extremely dim,

C/ You see nothing on the screen.

Try the above, let us know what you see from the above, and we'll take it from there.

Good luck.

EDIT

You replaced the LCD or the touch screen?

One last time, if your iPad is still not sealed, try IN THIS ORDER: disconnecting the battery then the LCD, then reseat the LCD connector and finally reseat the battery connector, then restart the iPad. Any change?

Here are the possibilities:

1/ Your iPad needs power cycling = disconnect then reconnect battery;

2/ Your screen needs to be reseated (unlikely in your case),

3/ During initial repair you damaged the LCD or if you were replacing it you could have received it defective;

4/ You damaged the back-light circuit during the initial repair (in which case you should see a very very dim image on your iPad, or a very dim Apple logo during restart/reboot. Try rebooting again and look very hard);

5/ You damaged something else on the board during the initial repair.