Kies is flaky, but needed to download the drivers to work with USB. Though you can find other sources that have it, but run the risk of viruses if not from a trusted source.
Try to completely power off the computer, wait 30 seconds and power it back on. See if that helps. If not, next step would be to uninstall Kies, do a reboot, reinstall Kies and do another reboot. Hopefully that might help.