I ran into the same problem...on multiple computers and with multiple cables. It worked, and then suddenly it didn't. No matter what I did (Kies mini, Kies full, drivers, etc...), nothing worked. I was sure that it was a problem with the phone.
Yesterday, I went to Radio Shack and bought a new cable. The brand of the cable is "Gigaware". Plugged it in to my phone, and then my computer, and I was back in business!
I thought it was strange, because I tried at least 4 different cables and 2 never worked, the other two (ATT branded) did work, but then suddenly did not.
I think the connection at the micro USB slot on the phone is very sensitve and will not work if everything is not perfect.
So, do yourself a favor and spend the money (I spent 30 bucks) and buy a good cable. That could fix your problem the way it did for me.