black screen but itunes says iPhones on

i got this iphone used and it wasnt working so i thought it was just the screen so i replaced it but it didnt do anything so i plugged it into my desktop and opened itunes and it said allow access on iphone so i thought is something bad on the logic board its not the black light but it doesnt vibrate either so is the phone not on or is it it just doesnt respond i think its the connector or something like that anyone have any ideas and yes i put it in dfu mode and all that stuff so no holding power and home down wont work