Help Screen goes black

Morning all.
My wife managed to drop her xperia u on the road and it got run over before she realised it was on the floor.
It bent the plastic back and smashed the digitizer so thought I was lucky.
Anyway I've replaced the digitizer and back, started it up and updated a few apps but it keeps turning the display black. It will sometimes load completely up before doing it but usually doesn't even bring up the Sony start up screen. The phone's not been rooted by the way.
Any idea what it might be?