This is kinda just a guess, but I have noticed that with a USB device and/or memory stick that sometimes you have to try different USB ports on a computer before one of them will actually recognize that you have inserted it into the computer.
So perhaps try all the USB ports on your computer, or even on a friend or family member's computer or laptop and hopefully you will be abler to find one that will allow you to re-format your USB Drive.
http://www.sandisk.com/about-sandisk/contact-us/
But if still no luck, perhaps you should try contacting SanDisk customer support and they should be able to help you.