I do not see arduino uno R3 in device manager. Win7 and Win8.1

Hi,

This weekend I try to connect my uno to the usb. but windows does not play a sound for new device connected. I try to connect on other computer but there is the some problem. There is no change in the device manager. The arduino power ligths are on. I try to put the arduino in DFU mode but nothing has changed.

Can any one help me or say is my uno broken.

thanks