Make sure you have Samsung Kies or the Samsung USB Driver installed on your PC.
Download Odin3 3.04.
Download and Extract Param_Bin_I9305 - Download - 4shared to your PC.
With your Phone Switched Off, press and hold: 'VOL DN' + 'HOME' + 'POWER' to enter Download Mode.
Connect your phone to your PC via USB Cable and start Odin.
Check your phone is connected by seeing if a COM Port in a Blue box is present under ID:COM.
Press the 'PDA' Button and select the 'Param_Bin_I9305.tar.md5' file you extracted from the Zip Archive earlier.
Press the 'Start' Button and wait for it to finish flashing. If successful, you will see a Green Box with 'PASS!'.
Wait for your phone to reboot then go to your Dialer and press: *#06#. You should now see your IMEI and your Data Connection will be back.