Cannot boot Latitude laptop

I have a Latitude laptop that won't boot. When I try to  boot it, I get a message that the system can't find a bootable drive.  I have a bootable USB,  but I can't change the boot order to check for a USB because the arrow keys won't work on the screen. I also have an .isoo drive I downloaded from  Microsoft on a USB, if that helps. How do I proceed to boot the system?