rooted my phone but want to go back to stock rom
Samsung Kies is no longer recognizing your phone because you've replaced the operating system by installing a custom ROM (CyanogenMod in this case.)  Simply rooting it and leaving stock Samsung firmware would've been sufficient, and wouldn't interfere with Kies.  You should have stopped after the "Install SuperSU" step.

With that said, here are the instructions for returning back to stock Samsung firmware:


Galaxy Express comes in different flavors, depending on your country and/or carrier.  Find out the exact model number of your device (e.g. GT-I8730/T for international version, or SGH-i437 for US AT&amp;T/GoPhone) and download the latest firmware from here. Make sure to pick the correct region.
Make sure you have taken a full backup of the contents of your device. Flashing factory firmware will wipe your device clean.
Extract the downloaded zip file to your desktop. You should get a file with a .tar.md5 extension (.md5 extension may not be visible in Windows)
Download ODIN (instructions here). This is a special tool used to flash firmware files on to Samsung devices. Extract the downloaded zip file to your desktop
Make sure you have installed all the necessary device drivers for your phone on your PC. You can also get the drivers by installing Samsung Kies
On your phone, enable USB Debugging from Settings -> Developer Options
On the PC, launch ODIN. Click the PDA button in ODIN and load the stock 4.1.2 firmware .tar file you got in Step 3
Make sure everything else in ODIN is left at default values. Just make sure Auto Reboot and F.Reset Time boxes are checked, and the RePartition box is left unchecked
Switch off your device and reboot it to Download Mode by pressing and holding Volume Down+Home+Power buttons together. At the warning screen, press Volume Up to enter Download Mode
Connect your phone to the PC via USB cable now
ODIN should display the message Added! and the ID:COM port box in ODIN will turn yellow
Click the Start button in ODIN to start flashing the stock firmware on to your device.
Once ODIN has successfully flashed the file, it should display the message PASS!, and your phone should reboot automatically
Disconnect your device from the PC now, and proceed to set up your phone.


After the above process is successfully completed, you can try rooting your phone again.  This time do not install a custom firmware, and you should be all set.