I've killed my Samsung D500. What have I done?

I was given a Samsung D500 with a cracked screen, which I stupidly thought I could replace myself. I took the whole phone apart before realising all I had to do was prise off the front housing which covers the glass. Having reassembled the phone, it now won't start at all. I have disassembled and reassembled the phone several times and still no good. Is it possible I have damaged the connecting ribbon in some way, or not replaced the ribbon correctly each time, or what else could potentially cause this?