Screen stuck on apple logo

Customer brought Iphone 6 plus to me today saying that after she dropped her mobile on car seat it went dead despite plenty battery charge. She then tried charging the apple logo appeared but stayed on and will do nothing else.

When you do a soft reset the screen briefly shows a section on bottom half that is a different colour. I have also seen a completely purple screen on another reset. There has been no water damage.

I have tried a hard reset and disconnected all hardware off pcd incase a faulty camera etc was causing this issue. I also tried different screen but still no joy.

I have not yet tried another battery as I do not have one.

Any ideas as to where to start on this one?

Seen something similar before?

Any help or advice hugely appreciated

Thanks.