@Ambucias is most probably right. However, I'd like to ask you if you've checked if the USB flash drive is recognized in Disk Management? It's possible for its partition to have been corrupted and thus not appear in Windows Explorer (This PC, My Computer). If this is the case, you could try reformatting it, to see if you'd be able to use it properly afterwards.
I don't want to get your hopes too high, but it's worth the try.
Hope that helps. Please let us know how it goes.