Here is a quick example of grub commands that might just work, explanations and caveats below.

grub2

Most likely for post-2010 installs.

set root=(hd1,1)
chainloader +1
boot

grub

Most likely for pre-2005 installs.

root (hd1,0)
chainloader +1
boot

For the 2005-2010 period, your guess is as good as mine, but if you use the command for the wrong version, you only get a harmless syntax error on the first command.

Choosing the right root

At startup, grub will probe for your devices and assign numbers to them. All devices that are partitioned (hard disks and flash drives) will also have numbers assigned. The format is (<deviceName>,<partitionIndex>). In grub2, partition indexes changed, so the two examples above have the same effect despite looking to use different roots.

Your first device (hd0) is whichever device grub just loaded from. After that, you can usually assume that all the internal devices will come before your external devices. They will most likely be in the form of hd and a number.

After the comma is the partition index. Hard disks and thumb drives will almost always be partitioned, so you must choose the right (and most likely only) partition. CD-ROMs are usually not partitioned.

More documentation: http://www.gnu.org/software/grub/manual/html_node/Device-syntax.html

When choosing your root partition, you can use the Tab key to probe for device names and partition indexes. Just open parenthesis and start pressing Tab to see the list. Alternatively, newer versions provide the ls or find command.

Go to your BIOS menu, and look for anything that has UEFI in it, and change to "legacy", if your BIOS supports that, most BIOSs should support that.
Also did you make sure the USB you are using is bootable??
run (on another system) "sudo fdisk /dev/sdX" where sdX is your drive's node, then issue p, and see if you have the bootable flag next to any of the partitions.
If you are using UEFI/GPT, fdisk won't work for you, but parted/gparted will.