did you add a custom recovery to your phone (i.e. CWM)? If so, hopefully you made a backup, so all you need to do is restore it.

If no recovery, then you probably need to install ODIN on your pc and to return to stock. follow the thread here

ODIN return to stock

be sure just to flash the 'pre-rooted' tar file.
after that, install the custom recovery. directions here

then make a backup of your phone just in case something like this happens again.