Hate to break it to you, but restoring a nandroid isn't that likely to bring your IMEI back. If you're very lucky, flashing a stock rom for your country/carrier (if you're already running one, flash others) might bring it back if it's a case of temporary IMEI loss. If not, many mobile repair shops (not Samsung service centres) should be able to write the IMEI back to the device relatively cheaply. And if not, a Samsung authorised repairer can do it (or they may replace the motherboard which has already been suggested to you), but that can obviously be somewhat expensive.
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 ?