1*First try your device on some other system. There may be chance that it is problem with your system. Like problem in OS, system port.
2*Unplug and plug can also fix this problem. sometime, the cable is not properly attached.
3*Start -->> Run -->> type devmgmt.msc -->>OK -->> Expand the Universal Serial Bus controllers node, and then follow these steps for each USB Root Hub that is listed:
Right-click the USB Root Hub, and then click Properties.
Click the Power Management tab.
Click to clear the Allow the computer to turn off this device to save power check box, and then click OK.