I'm assuming this USB drive worked before and was assigned a drive letter? Did you check to make sure that it was not assigned a different letter this time?
You should look in Device Manager for any conflicts with your USB ports. See if you have a yellow question mark or exclamation point under the USB section. You can also right click on Universal Serial Bus controllers and select scan for hardware changes to see if it finds it.
Another thing you can do is go to a href="/support-categories/hardware/476-disk-management-drives" target="blank">Disk Management and see if your drive is listed there. If so you can assign it a drive letter if it doesn't have one.
I would also try this drive on another computer to make sure there isn't a problem with the drive itself. There is a chance that it has gone bad. Also make sure that your USB ports are working by plugging in a different USB device into the USB ports to make sure it is recognized by Windows.
Check and see if you have the USB device icon by the clock in the taskbar. Double click it and see if your device is listed in there. If not it may be another sign that it has gone bad.