Start flashing different stock roms for your country/carrier (I presume you don't have an EFS backup); if your IMEI doesn't return after flashing a few of those, it won't be coming back. Take it to a local mobile repair shop in that case, many have equipment to re-write the IMEI back to the device fairly cheaply.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?