Why Won't it come on after repair?

Three weeks ago, my phone randomly stopped working. It was minding it's on business sitting in a corner and all of a sudden the screen stayed black. I could hear the notifications but it wouldn't turn on(the screen). Then I charged it and when I would try to power it on, it made the power up sound and vibration but didn't light up. No lights would light up. I was told it was my LCD so I ordered a new part, replaced it and made sure the ribbon cable was secured and I still got the same result. Now if I try to turn it on, the power up sound keeps replaying every 10 seconds til it dies, or i disconnect the battery, still without and sign of life in the new screen/LCD or even notification lights. What could be the problem?

NOTE: It was in some heat that day, but it's always hot here. The screen had a crack and some small glass chips that had fallen out from the top left corner from a previous drop months ago, but other than that it was perfectly fine.

Thanks so much for any advice!