Cant boot PC via USB

I have a Toshiba satellite u840 laptop I had windows 10 running on it, until I had some problems and had to re-install win 10 but switched on fast boot and I could not boot into win 10 .Afterwards I was finally able to turn it off but now I can't boot into win 10. And get a underscore(_) on the top left corner of my screen.

I then tried to use a bootable USB to install win 10 but I when I try to boot it via the usb it just shows a black screen

Please help