Grub2: boot USB from GRUB prompt

When the PC will not boot from USB, you can use GRUB2 to boot from
the USB. 
 In the GRUB menu: Press 'c' to go the GRUB prompt 
 grub> ls 
 (hd0) (hd0,msdos1) ... 
 grub> set root=hd0,msdos1  
 grub> chainloader /efi/boot/bootx64.efi 
 grub> boot 
 The USB boot menu will appear. 
 The above is of course an example, every situation will be different.
But the idea is the same.