Ah, yes the dreaded Toshiba. At this point you have exhausted all troubleshooting we would refer to concerning the type of problem you are facing. I, too, Sir have had some Toshibas, die on me, and no matter what I replaced, they would never boot. Try to boot and press F2 or F8. see if it responds. Also you may need to try the ESC key on some models. If it boots to the BIOS let us know!
Ok so you have changed the boot order in the bios to the USB ports? And you have a bootable ISO on the USB?