Has anything happened to the USB flash prior to the error that you get? Have you tried openning it on another PC or another USB ports?
Please, try these commands:
1. Open command prompt
2. Type the following:
attrib -s -h -r X:/*.* /s /d
Then hit "Enter" and wait about for some seconds
Be careful! Respect the spaces between words and script. Change the letter X with that has been assigned to the USB device on your PC /that you can see it in My Computer. It can be anything, depends on each computer./