Try to include as much detail as possible. Are you on a PC or a Mac? Are you using the OEM cable (the cable that came with the phone) Have you tried switching to another USB port. Make sure you coming from a USB port directly from the PC (assuming you have a Windows PC) and not a USB Hub.
Kies has a "Trouble Shoot Connection" button at the bottom try it. And in a worst case scenario uninstall Kies and hook up your phone, let windows find the drivers then re-install Kies, disconnect your phone and then try it again.