Why? Probably because you didn't "safely remove" or "un-mount" it. When you just pull it out of your usb port without "safely removing" it, eventually it will stop working.First try another USB port. Then try un-installing the driver, then re-installing the driver. Then try another flash drive.Good luck.Also, if you were "safely removing" it, flash memory doesn't last forever. In that case, get a new one.