Go to settings, press "more.." select USB Connection, press button, then plug in phone to pc.
Also go http://www.samsung.com/us/kies/ for Kies that bypasses all that. If that doesn't work then replace your cable with a "data cable". You also might be using a charging cable which doesn't have data support connecters in it.
This one will work:
I know it says for HTC, but its for all micro usb ports.
My computer will charge my phone but will not sync. I am using the charging cable, the phone is a little over a year old, I've used that cord since the purchase of the phone and have not had a problem until about a month ago.
And Kies for whatever reason will not work either, it will not finish loading, indicating a problem with the USB and i have tried a different USB cord as well.