This issue is most often caused by a poor connection between the camera and the computer. You can try the following steps to fix it.
Turn off your camera.
Take out the camera dummy battery.
Plug the dummy battery back into the camera.
Plug the USB cable into a different USB slot on your computer.
Make sure all the connections are secure - Both sides of the power cable & both sides of the USB cable.
Turn on Camera and restart computer.
If that still does not solve your issue, try switching to a different USB cable and/or different power cable & dummy battery for the camera.