If a USB device isn't working, make sure all USB cables and power cords are plugged in properly. Also, make sure you installed any software that came with the device. You may be able to download the software from the manufacturer's website.
If the device doesn't have a power cord and is plugged into another USB device that doesn't have a power cord, try plugging the device directly into your computer or to a USB device that does have a power cord. You may have to unplug and replug the other device as well, if it has stopped responding.
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