Is your computer a laptop or a desktop?
If it is a laptop, check if there is an "F" key that can be pressed before disk boot and give you the option to select the booting device.
If it is a desktop, check in your BIOS for options like "boot order" and "hard disk priority" with your USB drive plugged in. For example, my kids' desktop PC has a Gigabyte mainboard in which I have to set USB-HDD first in boot order and also select the specific USB drive in Hard Disk Boot Priority so I can boot from it successfully.