Sounds like it's either dead or has a virus. There are some specific virus's just for USB drives. See if you can get a Anti-Virus engine to scan it. Also they do have a limited life. So if you were using it for something that is write intensive a cheap drive will fail pretty quickly.

"most commercially available flash products are guaranteed to withstand 100,000 write-erase-cycles for block 0"
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Can a USB thumbdrive "wear out"? - Ask Leo!