Does it still attach as an MTP device, i.e. can you still access it from the computer?
Assuming the answer's no, then it could be MTP was unselected and needs to be turned back on (in vanilla Android: Settings .. Storage .. Menu .. USB computer connection).
Otherwise, the most likely issue is a dodgy USB cable - try a different one.
If not, then it will be drivers:
- disconnect the phone from the computer
- uninstall ALL phone related sofware (Kies) and drivers from the computer
- restart the computer
- restart the phone
Re-install Kies - do NOT connect the phone until prompted.
NB: do NOT disconnect the phone until ALL drivers have successfully installed
