Black screen after replacement

When you say you replaced the screen. Did you replace just the glass? Or the entire screen assembly?

Also, where did you get the part? I've seen many cheap screens for iPhones from Hong Kong and China have bad LCD's right out the gate.

If you were trying to replace the glass only and attempted to separate the glass and LCD, then I would have to say the LCD was damaged in that process and that's why its no longer kicking on.

If you did buy the entire screen assembly with the LCD and Digitizer already fused together, then its possible the connection was not properly made. I would make sure the LCD connector is plugged in snug. Also, try and hard reset. (Hold power and home button for 10 seconds until phone restarts). If the phone was kicked on by mistake during the repair, then it may just need a good hard reset to fire up that new LCD.

If none of that works then its possible the LCD in the new part is bad. Contact the supplier that you ordered it from and see what they can do to replace the part for you.

Hope this helps!