1) Remove your IMEI from your post immediately. You obviously don't realise how easy it is for someone to take it & write it to another device. This will result in your IMEI being blacklisted by networks around the world & your phone will become a plastic paperweight.
2) Try flashing a stock rom for your carrier/country, sometimes the IMEI disappearing is a temporary thing. Also, if you have an EFS backup (and you have no excuses for not having one if you're modding your phone in any way), try restoring it. There are also a lot of 'guides' on here that can be found by searching which claim to enable you to recover your IMEI. I don't know whether any of these work personally or not as I've never had to use them (I make backups).
3) Failing all that, take it to a local mobile repair shop. Many have the necessary equipment to write the IMEI back to the advice fairly cheaply (for example, I can have this done for ~AUD$30 where I live).
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 ?