Do you want to get back your data on this USB drive?
If you do not, the way is really easy: you could download format software to help you on the webs.
If you do, you should firstly try a USB drive recovery freeware to save your data back and then format it with the help of format software. (To tell the truth, my friend has used this freeware to recover his data from a WD external hard drive. The result is great! You should try it! You could visit this freeware there: blog4mark.blogspot.com/2013/01/unformat-memory-card.html )