No display after changing screen
Replaced smashed screen, display come on then went off & won't come back on, still hear sounds on it
Best bet is to reseat the connecting cables. If that doesn't work, try holding the power and home buttons for 9 seconds. If that doesn't work, put the device into DFU mode and restore to factory settings. If that doesn't do the trick, might need to test another screen, preferably a known working one. If the test screen does not work, then the issue is likely with the logic board. If the test screen works fine (most likely scenario) then you'd just need to replace the screen.
Try these steps,, first disconnect your battery, with your battery disconnected you now can disconnect all 3 screen connections, reconnect them and be triple sure they plug back in correctly, they should click or make a snapping noise when plugged in or most likely they are not plugged in right, now either plug in the battery and try to turn it on or with the battery unplugged put a charger into the charge port to see if the apple shows up on the screen, if none of this works you probably bought your screen from amazon or ebay and its faulty so you need a good 1, let me no if this worked please, firstname.lastname@example.org