Why won't my screen power on after replacement?

I've replaced my screen before and had no problem everything worked great! Now I'm doing the same thing and now my phone would not power on at all! I do remember I messed with the "Power/Volume" flex cable and the "white plastic" piece came off ... I don't think that should affect it... but if I does please let me know.. and when I connect the phone to the charger... the white LED comes on but still no screen... and I did recheck all my connections ... please HELP!