Would you believe it - the same thing just happened to me!!! I had the phone connected to the pc, charging, when I booted up. But then undid the cable later and carried on using pc.
1. After your post was answered, I decided to use KIES to search for a way to transfer a sound file so connected phone - nothing. Opened KIES manually, but device could not be found.
2. Rebooted pc - nothing
3. Rebooted Phone - ah! KIES was opening on the pc
4. Connect device - AGH!! Tried trouble shooter - it worked but did not connect device
5. So I tried Troubleshooter again, noted that I got a pop up telling me device driver not installed, so was about to three finger salute and stop KIES but decided to let it run the trouble shooter program.
6. Was about to give up and uninstall KIES windows came up telling me it was installing device driver. It searched net, nothing, then it began to search preconfigured folders and came up with device driver installed and ready to use.
7. So the fault is in windows. (Win 7 Home Prem). I plugged phone back in and KIES (already open) began to install device. It worked.
The computer could not see device because it had somehow uninstalled driver