Display assembly unresponsive after replacement

If the phone is completely unresponsive, you might want to go back and double check your steps (follow this guide) to see if you may have caused any accidental damage to the various flexes. Using magnification helps as it doesn't take much to cause issues.

Another way to test the phone to see if it is logic board related or not is to disconnect everything (screen, cameras, buttons) except the lightning port and battery (disconnect the battery first, then reconnect just before you test). Then connect it to an iTunes enabled computer. If the computer recognizes the phone, that means something you disconnected is causing the issue. Re-connect the modules one by one until you figure out which one.

If the phone is still not recognized, it is most likely the logic board although it could still be the Lightning Port or Battery. Depending on how much time and money you want to invest in troubleshooting, you could order replacement parts or have it looked at by a repair shop, preferably one that does micro-soldering.