EFS backup ? Restore it. How ? Search. Many, many, threads on the subject here.
No EFS backup ? Get some stock roms for your country/carrier from Samfirmware & start flashing. You may get lucky & the IMEI loss may be temporary. If that doesn't bring it back, many local mobile repair shops have equipment that can re-write the IMEI back to the phone fairly cheaply.
And finally, if you want meaningful help rather than generalisations, you're going to have to provide a lot more information (this goes for future threads you make as well). Without rom, kernel, Android version, rooted/not rooted, exactly what you've been doing to the phone lately you limit your chances of getting help beyond what I've offered. We shouldn't have to ask you this stuff/engage in 'pulling teeth' to extract it from you.
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 ?