Why is my iPhone 7 plus unresponsive after hard refresh?

So I’ve had coloured vertical lines on my iphone screen for a while. Then it all went white the other day and became unresponsive. Nothing would work, touch screen unresponsive, so tried a soft reset. Nothing. So tried a hard reset. Screen went bank as expected, but then the Apple logo never appeared and now it’s totally unresponsive.

I’ve cleaned the LCD flex cable sockets to see if that made a difference to the coloured streaks, but my phone still won’t turn on.

I’ve tried all the tricks, tried to hard refresh again, charged it for a couple of hours, plugged it into computer to reboot, but nothing.

Any clues as to how I can power it back up again?