Bad sectors on the flash drive forsure. Perhaps you pulled it out when it was not ready, this usually screws *** up on a fat32 formatted Flash Drive. Do as jsmakkar said, and if needed use the recovery program he linked too.
Long Path Tool is very useful if you are having problems in deleting, unlocking, copying and even renaming files that are considered filename too long by your system. Yes, these problems can occur even while using the latest Windows Explorer or FAR in managing your files. This tool can help you simplify files names that are categorized as filename too long by your system.