Hi Justin,
I'm curious as to whether you used the iFixit guide for replacing your screen or not. Going over the instructions, it seems there are steps missing with regard to disconnecting the battery. Here's the guide I was looking at; it's the "Featured" one.
iPad Pro 12.9" 2nd Gen Screen Replacement - iFixit Repair Guide
Anyway, if that's the guide you used, the instructions for unplugging the battery are mysteriously absent. On Step 26, it does have the following warning.
It's important to disconnect the battery before you proceed further. Detaching or connecting the display cables with the battery plugged in can cause a short that will destroy touchscreen functionality.
However, there are no other references to inserting pieces of a playing card under the battery connector to cut power to the display prior to replacing it. Hopefully you caught that in your replacement process and did disconnect the battery, because as the warning clearly states, unplugging the display cables with the battery plugged in can cause fatal shorts capable of killing your screen.
So if you go to the iPad Pro 12.9" 2nd Gen Battery Replacement Guide, jump to Step 28 and follow those instructions through Step 31 before unplugging the display, and don't reconnect the battery until after the display is fully reconnected.
Anyway, keeping our fingers crossed that your screen isn't toast, what we want to do is go double-check your work. Those long FPC connectors can sometimes be tricky to get plugged back in, so it's entirely possible you just need to reconnect those plugs and it could fix the problem. So follow the guide(s) and get the display unplugged again. Use a magnifying glass and a bright light to check the pins on both connectors since we don't know which is the digitizer and which is the display, and look at both sides; the one on the motherboard and the one on the flex cable. Verify there aren't any bent or broken pins there - bent pins can sometimes be straightened with fine tip tweezers.
Once you're happy the pins are okay, clean both sides with 90% or higher concentration isopropyl (rubbing) alcohol, give it a minute to dry, and carefully reconnect both display connectors. Again, pay attention to the caution note on Step 30 of the screen guide.
To re-attach press connectors like this one, carefully align and press down on one side until it clicks into place, then repeat on the other side. Do not press down on the middle. If the connector is misaligned, the pins can bend, causing permanent damage.
Make sure they're both in securely, then before gluing everything back together, reconnect the battery and see if it's working. If it is, button it all back up and congratulate yourself on a successful repair! Otherwise you'll have to figure out if the new screen is broken or defective; at this point the best thing would be to try plugging the old screen back in and see if the backlight works on it. If it does, you've likely either blown the backlight circuit on the replacement screen or just plain gotten a defective display.
If it doesn't work, that could mean problems with the backlight circuit on the logic board, the repair of which is beyond the scope of myself and iFixit's guides.
So try reconnecting the display then come on back here and let us know what you find and we'll figure out what your next step should be.
Just wanted to let you know that I've updated the guide to put the battery disconnect information back in. However, the change has to be approved by iFixit, so until that happens you'll see a notice at the top to this effect:
This guide has more recent changes. Switch to the latest unverified version.
Click on the link to go to the one I've changed.
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