turn on your DDJ 800, connect it to the PC (which ever you like) and take a look at your device manager, see if there's a yellow exclamation mark next to any new hardware.
Keep an eye on it, disconnect the 800 and see if it disappears.
If it does, you can boot into safe mode and remove it, restart and reinstall.
Also, check the USB cable by swapping it with another. It could just be that causing the problem.