TestDisk. Long time since I did DOS and stuff. The hard part was finding an idiot proof set of instructions how to use Testdisk! The nearest I ,managed was at SysAdminHell a post titled Awesome Utility: TestDisk. I suggest you download TestDisk and then go to the instructions at SysAdminHell.
I found placing TestDisk on another USB Drive worked best for me. After a bit of trial and nor too much error I managed to get the USB drive to boot. I looks as though the FAT32 Boot sector was corrupt. BEWARE of NOT EJECTING your USB Drives!