Did you disconnect the battery during the screen replacement process? Need to be careful of Static Discharge, which is something people forget. A small amount of discharge can easily damage the phone.
The other possibility is the LCD/Digitizer may be defective, unless you have another unit to test it on.
Last option is the board, whether there was a shortage or not, if the phone was dropped there is a chance that the boards power system may be at fault. So if you turn on your device and you can hear it, you know the screen works, 100% tested, then power may not be properly distributed.
Good luck, wish the best